God-believers have killed over 40 million Hindus in the sub-continent. God-believers have killed an estimated 500 million non-believers over the course of Islamic conquest in an attempt to bring people under dar el Islam (spelling, I'm sure).
Islam is unlike all other religions in that it requires adherents to fight and make the world Islam. The golden rule doesn't exist in Islam. There are no human rights in Islam unless you're a Muslim. But Islam is a religion, religion isn't Islam. Ancient conquests of enemy territory was hardly unique to Islam. Political conquests have always existed, only a fraction of which were led under a banner of religion. Religion can be used to unite a group to serve a common cause. Nationalism and race can also do that.
So, I'm not sure you're entirely accurate to state, Dartagnan, that atheist leaders are more likely or more inclined to commit mass atrocities.
What I've said, time and time again, is that as a percentage, this is a hard fact. And this logically means that an atheistic dictator is more likely, based on history, to engage in genocide. You point to the few medieval Islamic caliphs and I can counterbalance that by pointing to a hundred theistic Presidents, kings and princes who didn't commit such atrocities. On the other hand, I can point to a half dozen atheists from the last century who did commit these atrocities, but can you counterbalance that with hundreds of atheist rulers who didn't?
I think it's ideology which drives this one, more than anything.
With Islam, yes. But misplaced ambition and a hunger for power has always existed in humans, especially those in a position to wield it. It is a temptation and is something unique to humans in general, not just religious humans. Those who wish to accomplish these selfish goals can use whatever they want at their disposal. Religion is just one of many convenient and effective social mechanisms that is used to unite a group under a common cause. Nationalism and race can also serve that purpose. It isn't surprising that those who used religion for political means, weren't really religious (i.e. Hitler, Constantine) at all. I happen to believe Muhammed used religion for his own means the same way Joseph Smith did. It was for their own personal gain.
The problem is human nature. Religion at least offers some sense of moral grounding whereas atheism offers nothing of the sort. If anything, it has a tendency to ridicule morality because it is strongly associated with religious teaching. Beastie once tried to say following the "scientific method" would solve all these problems in human society, as if it could somehow provide a better substitute for all the positives religions provide. She never did explain how this would be true.