I usually switch from good to evil... if my current charater is good, then my next will probably be evil and so forth... I'm currently at the evil side and having a lot of fun.
My character - Falador - has passed the point of no return way back if there ever was any and I don't see him going to be bright side of life. He enjoys being a Dark child, a messenger of death, a servant of Sithis. There's nothing else he can do.
I don't think vampirism makes one necessarily bad.
Truly evil - most members of the Dark Brotherhood definitely, especially Gogron, he even admits killing a child.
Apart from them none comes immediately at mind, at least not truly evil.