Look, the fact of the matter is, the darkness = absence of light isn't how it's used in game.
This debate is quite a simple one. Can good people do evil things? And the answer is yes. Just because you are dark doesn't mean there is no light. Shadows are more defined with light nearby, aren't they? Yin Yang shows all darkness has light. The element of light is all about being righteous. Angels, golden dragons, it's all about being holy. Darkness is about doing what you have to. Screw the rules, that dragon of yours is trying to kill me, I'll steal it's health now.
The question eventually falls to this: Can good people do bad things? Can you be both a demon and a good person? Do you stop being a demon if you are a good person? Is the same true of devils?
And the answer is yes. Good people can do bad things. Using evil doesn't make you evil. Being dark doesn't make you evil. It just makes you dark. Science can't help debate a morale issue. Darkness ≠ evil.