Actually it's not; religion wins, hands down.
One word: Crusades.
Check it out. Still the top of the list for mass murder.
Just playing devil's advocate here (kind of), but consider the perspective of the Christian apologist:
The Crusades had nothing to do with Christianity, they were Church politics and, in that the course of action was in violation of tenets of Christianity, they were not Christian actions (though they were committed by Christians. Heh.).