Whenever we point out that atheist political leaders are guilty of the greatest numbers of mass murders in history, we get hit with a kind of "No True Atheist" fallacy. (While atheists try to "give" Hitler to Christianity, which is easily disproved despite their constant assertions, I do not buy attempts to "give" him to atheists, either. He was a nutty occultist who used religion for his own ends.) The excuse for the actions of atheist mass murderers is that the regimes are driven by political ideology.
So, it's just a coincidence that the greatest mass murderers in history were predominantly atheist? I think not. Especially since atheism has no consistent moral standard to tell them that torturing and killing Jews, Christians and others is wrong.