"'Evil' Paladins just doesn't work. It goes against the entire point of the Paladin. As mentioned above, 'evil' Paladins tend to become Death Knights instead.
Paladins get their power from The Light; from their faith and utter devotion to righteousness. As long as they believe, completely and whole-heartedly, that what they are doing is right, then they are empowered with The Light. If they started throwing diseases around and turned to darkness, then they can't really believe that they're doing the right thing, can they?"