Just wanted to mention my favorite evil villains from any game. Nephandi are just evil bastards, but have been turned that way. I'm guessing the concept was heavily influenced by the Hellraiser movies. Just go through a bunch of pain and terror, have all of your perceptions twisted so that you think all of existence is a lie and needs to be destroyed, and you'll be perfect!
Some very cool pages about the Nephandi:
White Wolf Wiki
World of Darkness Wiki
Two Nephandi and a Marauder
memoirs on a rainy day