From: Solfadore | #006
I would make a distinction between antagonist and villain. A character may be the story's antagonist, yet still be heroic. Similarly, the protagonist may also be the story's villain. Think Death Note, for instance - where Light would undoubtedly be the villain, and L clearly be the hero. Or American Psycho, where you could hardly qualify Bateman as something else than a villain. By and large, they would be antagonists, but not always.
A villain in and of itself is hard to define without running in circles. The character would have to be somewhat 'evil', to say the least, or have done 'evil' deeds. Morally gray characters are a bit iffy, though I guess we'll know when we see one. I guess it depends on the overall tone of the story. If it's a fairytale filled with goody two-shoes, then an morally ambiguous character who kills ten people to save thousands might be arguably seen as a villain. If it's, say, the Sopranos, the hitman who kills dozens of people but feels rather sorry for it might not be. Again, though, hard to define. Guess we'll know a villain when we see one.
As far as changing sides go, I completely agree with Kenshin. If a villain turns hero, or hero turned villain, they're eligible only if their time as a villain was longer or more memorable. Darth Vader would definitely be eligible as a villain, for instance. Severus Snape would not. At least that's how I feel.
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