Unless you mean that the reason is based upon religious doctrine. In that case, I understand not discussing it, especially if it is backed by scripture.
Got it in one.
I could, but I'm no where near as elegant, subtle, or authoritative as the text. I'd make a very poor substitute.
The basic idea is that there is really no difference between your thought life, your words, and your actions. They are all various ways you behave in a real sense and the modes and patterns of one inevitably influence the other. Thinking is not not a form of behaving even if it is hidden. So, if you change your actions you'd tend to pull your thoughts with it. If you change your thoughts, you tend to pull your actions with them. They are intrinsicly interconnected as who you are. The way that you entertain thoughts and emotions is who you are.
If you were transparent, if someone could see inside you, the fundamental ugliness or beauty of your thoughts would be revealed. Your actions are merely a reflection of this inner true nature. We don't look into a cup and see manure and slime and say, "The cup is clean.", just because the outside is polished. It is the same way with people. What comes pouring out of them is inevitably what's really inside them.
I should say that its not merely my particular religion that believes this. This is a fairly widespread philosophical belief which you might lump together as the 'spiritual school', and it includes everything from Stoicism to Buddism as well as the tradition I'm pointedly not directly discussing. (Note, I'm not saying that these schools of thought agree on all things, because they don't, but they do agree on this point.) I should also say that while there are a few points of belief where I'd be considered somewhat 'radical', this is not one of them. The idea that thoughts are every bit as evil as actions is about as mainstream and uncontriversial belief as you can get in my religion, so it wouldn't require any high degree of theological learning to track down the basis of this belief if you wanted to.
I'm struggling to communicate here in part because I'm being forced to jump back and forth between Good as D&D describes it, and Good as I see it. In D&D people are 'good' or 'evil'. As I see it, the concept of a literal 'good person' is ridiculous and self-evidently silly. But I'm forced to use the term both because people speak in that way in our consensus culture - 'he is a good person' - and because D&D describes things in that way. But as I use the term 'good person' only means 'relatively less depraved' and 'evil person' only means 'relatively more depraved'. I've discussed elsewhere what I believe alignment to be in this context, and what I suggested is that its not a shorthand for personality or even how someone behaves
all the time, but rather for how they tend to behave when under pressure, when stressed, and when they must make a hard choice.
So, someone challenged me when I said that there could be a murderer who was 'good', and said that was nonsense. But I didn't say that to excuse murder or lighten it in any way. It's hard to discuss this without grabbing some really contriversial real world examples, but I have no real problem with believing that there might be people who murder innocents without being fundamentally depraved (and hense, under my definition 'good people'). That's why considered the question of how the murderer behaves when they discover for themselves that they have actually committed murder so fundamental to determining 'good' or 'evil'. The question becomes, once your ignorance is removed, what do you do about it? The person challenged me with 'What if there was a good person who murdered evils because he thought they weren't people', and I responded with, "Well, what if there was a good person who murdered orcs because he thought they weren't people?" and that's not at all a hypothetical. It's meant to point out the many of our most heroic PC's are probably not that much better off than the 'evil elf child murdering person' of the example, and yet we want to insist that they are 'good'.
Now, I am an absolutist, so I agree that murder is evil whether you know it is evil or not, but that's not what I've been talking about. What I've been talking about is in a sense how you feel about doing evil. Does it bother you? Do you try to resist or avoid it? Do you feel remorse when you do wrong? That's really the only thing that divides the 'good person' from the 'bad person'. Because here is the clincher, I've said that you are in control of your own emotions, but in one sense that is false. No one is in perfect control of their emotions, and
in the same way no one is in perfect control of their actions. That doesn't make us less responsible, but it does mean that
noone can behave exactly as they would want to behave at all times. Go ahead and try it. You'll find I'm right on that.
So for me, the question of whether the person murdered someone one time or whether they blew up and yelled at their children in irritation is a fairly minor one on some levels. There are differences. Those actions have different consequences and they tend to indicate different levels of depravity (or they might not!) and the just punishment for those crimes might or might not be different, but the relative pardonability of the crimes or the 'evil' of the crimes doesn't differ. It's not like you act a little wrathful and you only have to do this little good deed to balance it out and then it is all ok, or if you murder someone you have to do a bunch of really good deeds to make up for it. We don't get to stand here and go, 'You are a bad person and I'm a good one.', and if we do its taken us very far afield from the empathy and compassion I see as central to 'Good'.
But anyway, that is taking us rather far afield. My original statement was that evil is just as confined in its behavior as good - and in fact in some senses it is more confined because the 'evil emotions' are more compelling to specific sorts of actions than the 'good emotions' and because good modes of behavior are more easily broken than bad ones. I stand by that. I think that my use of the word 'code' has confused things some, because as someone pointed out, neither good nor evil is intrinsically attached to the idea of an external reviewable code. In D&D terms, Chaotic Good would reject the idea of a code as niave or worse. However, 'code' or not, there is still a standard of behavior which is required of both good and evil, and the nature of the actor if trained in one mode of behavior or the other will tend to rebel against departing from that mode of behavior.
Frequently, in D&D I find I have to step back and impartially look at Evil and ask myself, "Why is that someone would choose evil over good? What attractive argument could be made in evil's behalf?" In our world, when appeals are made on behalf of evil, they are almost invariably said to be 'good' - very few people believe that they are 'evil'. But in the D&D world, where you can more objectively measure this, people wouldn't as often mistake themselves or what they believed for 'good' (although certainly many would), but rather they would tend to believe that Evil was the right, proper, and true way to believe. That is to say that they would believe 'Evil' (big 'e') was 'good' (small 'g') and 'Good' was 'evil' (small 'e'). There sense of right and wrong would be reversed, and they would tend to argue rather passionately (and if they were intelligent with great complexity and intellectual rigor) for what they believe was right (ei Evil). If you have a society that is say Lawful Evil, and knows itself to be, how do they justify themselves? They surely don't chuckle a maniacal laugh and say, "Muhahaha I'm evil. I worship the god of pederasty and sacrificing babies just because I love the terrific pain she inflicts on her worshipers, muhahaha."
No, people who hold evil beliefs will probably seem quite 'ordinary' and they'll believe something that rational and reasonable people could believe. They won't mostly be murderers, theives and robbers, which is why I find it funny when someone tries to 'detect evil' to discover the murderer. Because beside the fact that in my campaign the spell is likely to fail, you are also likely to find all sorts of suspects who aren't the murderer doing that. But they will be prone to, if they find that they can profit by it (and really, not many of us find ourselves in a situation where killing someone else is profitable) murder, or theivery or whatever, and most importantly they won't feel that they've done anything 'wrong' in doing so. They'll see their actions as perfectly justified, and they would do it again and they don't feel in the slightest sorry about it. But mostly, they'll manifest their evil in petty ways. It's not like you can divide the world into 'those evil murderers' and then 'good people' as if no one was evil unless they murdered someone or did some other horrible crime.
As for how all of this relates to alignment, first, I hope you can understand after all of that how I find it funny when people say that they find alignment confining and incapable of representing complex and multidiminsional characters. And secondly, and more to the point of why I'm passionately writing all of this, I don't evil cool at all and I very much want to destroy that whole idea. I think that the idea of evil being cool is one that is very very much not grounded in reality and reflects someone whose primary experience with evil in a conscious way (not their real experience with evil, but their experience with things that they labeled evil) was through movies with very unrealistic and dare I say one-dimensional characters. I agree with the symbolism in Tolkien, where his villains are so utterly lame that they lack bodies or even names and are empty shells or shelless emptiness. I think that the idea of evil being cool is pernicious and destructive.