The first thing that came to my mind was a fallen paladin. You'd be hardcore in this group of whackos. Plus, you'd probably fall into a leadership position. There are a number of specific routes you could go with this.

1. Paladin fallen to become Blackguard.
2. Paladin fallen to become Blackguard with some levels of Crusader or Warblade for mellee fun.
3. Paladin fallen to become Blackguard with levels of Cleric for Spellcaster goodness (er. .. evilness)