I like the idea that nobody actually thinks of themselves as being a villain. All "bad guys" think they have good reasons for what they're doing. I don't think it's an accident that a lot of the characters being mentioned in this thread are from comic books - the idea of "cartoon villainy" has been around for a while, and it's not a term that's usually used as a compliment. Do the villain's reasons have to be tragic? Not necessarily. But I think stories have a lot more depth when the villain's reasons are compelling, rather than just being evil-for-the-sake-of-evil.