"They're basically killing each other to see who's got the better imaginary friend."
-- Yasser Arafat, on going to war over religion.
Fighting over fundamental beliefs. As Kim Stanly Robinson liked to note in his Mars trilogy (especially in Green Mars), these kinds of people have more in common than not. They get stuck on points of order, and some are willing to die for those small differences.
Well, willing to die as long as they take more than one of the opposition with them.