Nobodies vs. the concept of good and evil
I don't think this entry will contain any new or shocking information. This is just a pseudo-essay musing on hearts, Heartless, and Nobodies; more than likely you've heard all this before. XD
On Hearts and the Composition of People
In the world of Kingdom Hearts, the 'heart' is very important to the worldview, and essential for life as we know it. But what exactly is a heart? While we can probably assume that the characters all have an internal organ that pumps blood through their bodies, that's not what's referred to whenever somebody in Kingdom Hearts talks about a heart.
The KH heart seems to be an almost entirely metaphysical thing; it can be lost, sacrificed, or regained, it can be overcome with darkness, or it can even be transferred from one person to another. Worlds also have hearts. It actually seems to be analogous to the popular concept of the soul, though characters in KH have souls as well as hearts. Loss of a heart does not exactly result in death, though it seems to be the end of a person - the by-products are a Heartless and a Nobody. So, I hypothesize that the heart primarily functions as the vessel of the personality and base emotions, the creative spark, and general "person-ness". That leaves the body for basic life functions, and the soul for higher mental functions and more complex emotions.
So in short, the composition of a person in KH seems to be thus:

Heart rules emotions, soul rules thought, body carries out life functions, and all together you have a person. That's what I've gathered, at least. Kairi pokes a bit of a hole in this, but Kairi was a special case.
Anyway.
The Creation of Nobodies and Heartless
So, we know the heart is important, and that losing it creates two entities - a Heartless and a Nobody.
The Heartless actually make a lot less sense as of KH2; in the first game they were easy to explain as the shells of people who had lost their hearts, but, whoops - the Nobodies take that honor. Thanks a lot for your misleading nomenclature, Xehanort.
It would seem, as of KH2, that the Heartless are actually hearts that have collapsed and been taken over by darkness, thus explaining the scene where the hearts raining down from Kingdom Hearts turn into Shadows. (Although wholly failing to explain why Sora was able to get his heart back. Did Kairi reinstall him from backup, or heal his collapsed heart?)
So, the collapsed heart is basically a walking mass of darkness. Because this violates the basic balance of light and dark in everything, it goes around trying to get hearts to fill its void; unfortunately, it only succeeds in collapsing the hearts it touches, and creating more Heartless.
Axel tells us that killing a Heartless with the Keyblade 'releases a captive heart', confirmed by the way we see a heart float away every time we kill one - but this doesn't totally make sense, because if the Heartless actually take and hoard hearts, then how are new Heartless created, if they are actually collapsed hearts? Maybe the Keyblade forcibly restores the heart to its whole state by reintroducing light, but lacks the finesse to do anything for the heart after that, thus it goes floating off to Kingdom Hearts?
I think I like that. We'll go with that.
At any rate, whatever the hell a Heartless actually is, it's there and is created when a person loses their heart (unless you're Kairi and therefore special). There is also something else created - a Nobody.
A Nobody is a little more straightforward, though their existence would seem to merit the term 'Heartless' more than the actual Heartless do. I believe it's Yen Sid who explains that the Nobody is the body and spirit left over when a heart is lost. In most cases, these simply degrade into the Dusks and other varieties that we see, but a very rare few (hi, Organization XIII) are able to keep something like their human forms.
Yen Sid also tells us that Nobodies, unlike Heartless, are sentient. This seems a little strange considering we have villains commanding armies of Heartless, but I guess it works. Heartless can be made to understand orders, while Nobodies already do. The sentience of Nobodies works with the idea that the soul rules over mental functions.
It is also alleged within the game that Nobodies do not feel. This is a fairly uncomfortable assertion, and seems to contradict evidence from the Nobodies' actual actions. Roxas' feelings when he doesn't know he's a Nobody - and also when he does - seem genuine. As do Axel's for Roxas, Demyx's in general, Xemnas's rage and despair, Larxene's schadenfreude, the Dusks frantically grabbing for falling hearts...
I think the best explanation of this is that most feelings do tend to originate in the heart; however, more complex emotions are more in the province of the soul, and the basic ones can leak over or be remembered well enough that an individual Nobody can trick himself into feeling them if he doesn't actually have them. I tend to think that the Nobodies are emotionally stunted, and the emotions they have might decay over time, and some of them really do just lack them - but in general, Nobodies are not without feeling. They're just limited, and perhaps muted.
At any rate, there we have it. Lose your heart, and it collapses and turns into a Heartless, while the rest of you turns into an emotionally-stunted Nobody.

The Philosophical Question
And so, in a roundabout way, we make it here.
Losing one's heart is a bad thing. It's a little hard to argue against this. Losing your heart basically robs you of your personhood; you have to have an exceptionally strong heart to even retain a semblance of your prior appearance as a Nobody, and you have to be unbelievably strong in order to retain your sense of self as a Heartless. (See Sora and Xehanort; and Xehanort seems to be a short-bus Special Case anyway.)
Also, losing your heart creates a Heartless. There is no way a Heartless can really ever be considered a good thingunless it's a White Mushroom; Heartless destroy hearts and thus make the owners of said hearts arguably worse than dead. Worse yet, they can corrupt the hearts of worlds, and thus plunge an entire world into darkness. Even one little Shadow, left unchecked in a world with an unlocked Keyhole, will eventually bring down a scourge of darkness and destruction ending in the demise of the world and a flood of hungry Heartless. They have no capacity to not do this; unless commanded, this is what they do. Only Sora!Shadow and Xehanort's Heartless did not exactly do this, and they're bad examples; Sora was a Heartless for about half an hour, and Xehanort's Heartless was just Special, and was still more or less following his destructive instincts.
So, losing your heart is a bad thing to happen, and the creation of a Heartless is likewise bad.
However, the process of losing a heart also creates a Nobody. And a Nobody has choices. A Nobody can choose just to wander alone in search of a way to restore his heart. A Nobody can even choose to resign himself to his fate; he likely won't, but he has the capacity to do so. A Nobody doesn't have to do anything bad. They are sentient, presumably can remember the moral codes they followed when they had their hearts, and can make their own choices.
It seems to be asserted in the game, mostly by DiZ and Yen Sid, that Nobodies are on the same level as Heartless; i.e. bad because of what they are. This is the most uncomfortable assertion made about the Nobodies in game; not least because you've just spent six hours getting attached to Roxas and Naminé and Axel when you first hear it. Sora buys into this, but has incongruous flashes of sympathy for Axel and Roxas, and even a brief one for Xemnas. But in general, he seems to go with the idea that because Nobodies are Nobodies, they're bad and need to be eliminated. Yes, he does go against this at times, but in my opinion, there simply was not enough doubt shown on his part for this to come across as anything more than hypocrisy on Sora's part; "Nobodies are bad, but you helped me/helped someone I know/are my Nobody, so you're okay". I love Sora to pieces, but this is still hard to swallow.
At any rate. The underlying philosophical question, which would have been really great to see better addressed in-game:
Are Nobodies bad because of what they are? Are they bad because they are by-products of a bad process? Is it fair to treat them that way?
Most of the fandom's answer is already "no", I'm sure. But it would have been wonderful if Sora had ever asked himself this.
On Hearts and the Composition of People
In the world of Kingdom Hearts, the 'heart' is very important to the worldview, and essential for life as we know it. But what exactly is a heart? While we can probably assume that the characters all have an internal organ that pumps blood through their bodies, that's not what's referred to whenever somebody in Kingdom Hearts talks about a heart.
The KH heart seems to be an almost entirely metaphysical thing; it can be lost, sacrificed, or regained, it can be overcome with darkness, or it can even be transferred from one person to another. Worlds also have hearts. It actually seems to be analogous to the popular concept of the soul, though characters in KH have souls as well as hearts. Loss of a heart does not exactly result in death, though it seems to be the end of a person - the by-products are a Heartless and a Nobody. So, I hypothesize that the heart primarily functions as the vessel of the personality and base emotions, the creative spark, and general "person-ness". That leaves the body for basic life functions, and the soul for higher mental functions and more complex emotions.
So in short, the composition of a person in KH seems to be thus:

Heart rules emotions, soul rules thought, body carries out life functions, and all together you have a person. That's what I've gathered, at least. Kairi pokes a bit of a hole in this, but Kairi was a special case.
Anyway.
The Creation of Nobodies and Heartless
So, we know the heart is important, and that losing it creates two entities - a Heartless and a Nobody.
The Heartless actually make a lot less sense as of KH2; in the first game they were easy to explain as the shells of people who had lost their hearts, but, whoops - the Nobodies take that honor. Thanks a lot for your misleading nomenclature, Xehanort.
It would seem, as of KH2, that the Heartless are actually hearts that have collapsed and been taken over by darkness, thus explaining the scene where the hearts raining down from Kingdom Hearts turn into Shadows. (Although wholly failing to explain why Sora was able to get his heart back. Did Kairi reinstall him from backup, or heal his collapsed heart?)
So, the collapsed heart is basically a walking mass of darkness. Because this violates the basic balance of light and dark in everything, it goes around trying to get hearts to fill its void; unfortunately, it only succeeds in collapsing the hearts it touches, and creating more Heartless.
Axel tells us that killing a Heartless with the Keyblade 'releases a captive heart', confirmed by the way we see a heart float away every time we kill one - but this doesn't totally make sense, because if the Heartless actually take and hoard hearts, then how are new Heartless created, if they are actually collapsed hearts? Maybe the Keyblade forcibly restores the heart to its whole state by reintroducing light, but lacks the finesse to do anything for the heart after that, thus it goes floating off to Kingdom Hearts?
I think I like that. We'll go with that.
At any rate, whatever the hell a Heartless actually is, it's there and is created when a person loses their heart (unless you're Kairi and therefore special). There is also something else created - a Nobody.
A Nobody is a little more straightforward, though their existence would seem to merit the term 'Heartless' more than the actual Heartless do. I believe it's Yen Sid who explains that the Nobody is the body and spirit left over when a heart is lost. In most cases, these simply degrade into the Dusks and other varieties that we see, but a very rare few (hi, Organization XIII) are able to keep something like their human forms.
Yen Sid also tells us that Nobodies, unlike Heartless, are sentient. This seems a little strange considering we have villains commanding armies of Heartless, but I guess it works. Heartless can be made to understand orders, while Nobodies already do. The sentience of Nobodies works with the idea that the soul rules over mental functions.
It is also alleged within the game that Nobodies do not feel. This is a fairly uncomfortable assertion, and seems to contradict evidence from the Nobodies' actual actions. Roxas' feelings when he doesn't know he's a Nobody - and also when he does - seem genuine. As do Axel's for Roxas, Demyx's in general, Xemnas's rage and despair, Larxene's schadenfreude, the Dusks frantically grabbing for falling hearts...
I think the best explanation of this is that most feelings do tend to originate in the heart; however, more complex emotions are more in the province of the soul, and the basic ones can leak over or be remembered well enough that an individual Nobody can trick himself into feeling them if he doesn't actually have them. I tend to think that the Nobodies are emotionally stunted, and the emotions they have might decay over time, and some of them really do just lack them - but in general, Nobodies are not without feeling. They're just limited, and perhaps muted.
At any rate, there we have it. Lose your heart, and it collapses and turns into a Heartless, while the rest of you turns into an emotionally-stunted Nobody.

The Philosophical Question
And so, in a roundabout way, we make it here.
Losing one's heart is a bad thing. It's a little hard to argue against this. Losing your heart basically robs you of your personhood; you have to have an exceptionally strong heart to even retain a semblance of your prior appearance as a Nobody, and you have to be unbelievably strong in order to retain your sense of self as a Heartless. (See Sora and Xehanort; and Xehanort seems to be a short-bus Special Case anyway.)
Also, losing your heart creates a Heartless. There is no way a Heartless can really ever be considered a good thing
So, losing your heart is a bad thing to happen, and the creation of a Heartless is likewise bad.
However, the process of losing a heart also creates a Nobody. And a Nobody has choices. A Nobody can choose just to wander alone in search of a way to restore his heart. A Nobody can even choose to resign himself to his fate; he likely won't, but he has the capacity to do so. A Nobody doesn't have to do anything bad. They are sentient, presumably can remember the moral codes they followed when they had their hearts, and can make their own choices.
It seems to be asserted in the game, mostly by DiZ and Yen Sid, that Nobodies are on the same level as Heartless; i.e. bad because of what they are. This is the most uncomfortable assertion made about the Nobodies in game; not least because you've just spent six hours getting attached to Roxas and Naminé and Axel when you first hear it. Sora buys into this, but has incongruous flashes of sympathy for Axel and Roxas, and even a brief one for Xemnas. But in general, he seems to go with the idea that because Nobodies are Nobodies, they're bad and need to be eliminated. Yes, he does go against this at times, but in my opinion, there simply was not enough doubt shown on his part for this to come across as anything more than hypocrisy on Sora's part; "Nobodies are bad, but you helped me/helped someone I know/are my Nobody, so you're okay". I love Sora to pieces, but this is still hard to swallow.
At any rate. The underlying philosophical question, which would have been really great to see better addressed in-game:
Are Nobodies bad because of what they are? Are they bad because they are by-products of a bad process? Is it fair to treat them that way?
Most of the fandom's answer is already "no", I'm sure. But it would have been wonderful if Sora had ever asked himself this.
no subject
It IS dangerous to start sympathizing with one's enemies, no question. Considering how Sora wants to Fix Everything, though, I'd think that if anybody could pull it off, he could. It would have been really hard to work, though, story-wise... Sora's the magic fixit, but the Org as a whole can't really be 'fixed'. Demyx could potentially be coaxed to join The Good Guys if he was sure they'd protect him, Axel already went turncoat for Sora/the Roxas in Sora, maybe one or two others could be convinced... but as a whole, they'd be awfully hard to 'save'. Some, like Saix, I don't think even could be 'saved'.
I guess what I'm getting at is I'd just have liked Sora to question things a little; even if he just came to the conclusion of "I feel sort of bad for you guys, but as long as you're doing this crap with the Heartless and hurting people and worlds, I'm going to keep fighting you and I won't hold back", I'd have loved that.
I totally agree with you that DiZ is a bastard. I can't help a little sympathy for him, but he and Xehanort basically fucked everything up and there is still no excuse for the way he treated Roxas and Naminé. Yen Sid, I don't know. He's cold, but he's also isolated. I can't really see him as a villain; just very out of touch. He has a lot of knowledge, but for all that he's a powerful old sorceror, he's running a little low on wisdom and just isn't that understanding of people and the way of the world anymore. His tower isn't literally ivory, but has had that effect, I think.
Oh, you're probably right about the hearts and memories. XD; But then, don't most Nobodies have memories of their past selves? I seem to remember Roxas being the weird one for not remembering Sora. ... Which was probably because Sora's heart got fixed real fast and the memories didn't have time to filter over. Okay, answered my own question!
As for your questions about the Heartless... You know what, man, I have no idea. x_x Your guess is honestly as good as mine, perhaps better. I've never thought about that.
no subject
On Sora's reasonings for killing nobodies, and the nobodies themselves:
I can't help but respond to the affirmation that Sora should have had better reasons for what he was doing in two ways:
a) Well, then the 'Land of Dragons', Pridelands and Hercules' part of the Collisseum should have been treated better or at least given flowers after their brutal rapes XD;;
b) Sora isn't that bright. Sora is a good person, but thinking is NOT his strong point-- he travels with two doofuses and of the two of them, it's GOOFY, who you would NOT expect, that is the smarter, the more observant, and the wiser in the end. Of the three of them, Goofy's your smartest character. Now I'm not dissing Goofy here, because I love his Milly-of-Trigun-esque penchant for being SUPER smart and seeming totally ditzy (or maybe not just seeming, whatever), but he's not exactly the smartest dude around. And he's smarter than Sora. So, while I want Sora to take that necessary step and be smart enough to question himself, his reasons, I think it's more likely for him to have reached that point after the end of KH2 than at any time during; after all, during KH2 he's still reacclimatizing to life several inches and at least a year older. (Namine says 'about a year ago Sora came to Castle Oblivion', so I assume it may even have been longer than that.) He missed a year of his life-- like Cloud in FF7, I see him as being more awkward and less able to think as deeply as he should because he was unable to develop at all in that year beyond basic physical changes.
And unfortunately, that's a pretty important developmental year, given the fact that your brain is becoming active in ways it may not have been before, too. Sora's not stupid, mind you, it's just that he was never the brightest of bulbs (just probably the sweetest, in the game, for he does seem to care about everything and everyone, for the most part, in spite of himself) and he's still very much a kid. He was naive in the first game, and remembers nothing of CoM-- a lot of sobering memories that might well have made him think harder about what he was doing in KH2. (Might well, hell. he'd have known who he was fighting at least a little, then.)
I'm with you in wishing KH2 had actually had the balls to tackle the wrongness of what was being done head on-- in a way what Sora was doing was almost like genocide, and we never have had proof that Sora's doing the right thing; those hearts he 'releases' apparently risk just becoming heartless again, and even if they didn't, who's to say it would be impossible to find a way to reunite heart and body to actually FIX people? Sora and Kairi did it without even actually having all the parts there (apparently, hence Roxas and Namine and all, you talked about it extensively already), and it seems weird that they should be 'special' in that they're the only ones who get that chance to be fixed. The fact that they CAN be fixed would suggest that Sora should be looking for a way to fix heartless, rather than effectively kill them or whatever it is he does (I can't tell if they actually die or just become heartless again, thanks to KH2. gah!), but that never occurred to him or anyone else either. Why not, I find myself wondering; but I'm willing to bet it hasn't occurred to most of them.
no subject
*several inches taller and at least a year older (sorry about the typo)
I didn't explain my comparison with Cloud well but I MEANT that Cloud was in a freakin' tube for five years and I totally blame that + stress + grief over Zack and hometown + trauma + MASSIVE trauma + some physical trauma to go with that mental trauma for his absolute WEIRD CRAZINESS. Sora's lucky it was only a year and under such good circumstances, I think. c_c but yes. That was, uh, what I meant with that comparison. It's totally 2:20. I should totally not be up.