Atheists like Mao and Stalin committed atrocities. But their atrocities were not committed in the name of?

ChristianPurity's response (oh so bright, as always) just begs the question of what caused the immorality of the people who caused the atrocities in the name of religion. Obviously, it couldn't have been atheism! Which just goes to show that had Mao or Stalin been theistic, it still wouldn't have guaranteed ethical behavior.