I've always assumed people make 'evil' characters for the same reason Sephiroth is so popular. Somehow 'evil' has become entirely associated with 'cool.' That's probably also the reason most evil characters are either dark colored, or black and red/neon. It doesn't really bother me, but I wish people would be a bit more creative if they're rping with an evil char. I mean, I've only ever made one evil char (hardly ever use him, so I doubt anyone's ever seen him), but he's not the "I am Shadow Deathblood, and I will kill ur family!" type. His name's Sascha, he's a smallish, unassuming off-white wolf, and he's pretty good at faking niceness until he can get someone alone and vulnerable. He has (somewhat) logical motivation for what he does. Course, I never use him because I've yet to find an rp with use for a subtle villain, but I keep hoping.
Anyway, my point is (sorry for the tangent) that it's not that hard to make an evil character original and semi-believable. It would be cool if more people did it.