Leaders invoke whatever they can to get the masses riled up enough to kill. The religion-free communist regimes were every bit as brutal as their religious counterparts. The British fought bloody wars under the belief that British culture was superior.

Some people have a psychological need to be a warrior for a cause. The actual cause isn't relevant - people kill each other over soccer matches. It was an evolutionary advantage when mankind lived in primitive tribes. Now it's just a clusterfuck. I don't blame religion for this behavior.