Every now and then around here, there's some sort of pissing contest between atheists and theists about 'which is worse - religion or atheism'. Of course, the usual suspects are rounded up such as the inquisition, the holocaust, Stalin, Mao, etc. Here's an interesting snippet regarding such 'rational' regimes and genocide:
It is true that there are millions of people whose faith moves them to perform extraordinary acts of self-sacrifice for the benefit of others. The help rendered to the poor by Christian missionaries in the developing world demonstrates that religious ideas can lead to actions that are both beautiful and necessary. But there are far better reasons for self-sacrifice than those that religion provides. The fact that faith has motivated many people to do good things does not suggest that faith is itself a necessary (or even a good) motivation for goodness. It can be quite possible, even reasonable, to risk one's life to save others without believing any incredible ideas about the nature of the universe.
By contrast, the most monstrous crimes against humanity have invariably been inspired by unjustified belief (emphasis mine). This is nearly a truism. Genocidal projects tend to to reflect the rationality of their perpetrators simply because there are no good reasons to kill peaceful people indiscriminately. Even where such crimes have been secular, they have required the egregious credulity of entire societies to be brought off. Consider the millions of people who were killed by Stalin and Mao: although these tyrants paid lip service to rationality, communism was little more than a political religion. At the heart of it's apparatus of repression and terror lurked a rigid ideology, to which generations of men and women were sacrificed. Even though their beliefs did not reach beyond this world, they were both cultic and irrational. To cite only one example, the dogmatic embrace of Lysenko's "socialist" biology--as distinguished from the "capitalist" biology of Mendel and Darwin--helped pave the way for tens of millions of deaths from famine in the Soviet Union and China in the first part of the twentieth century.
Source: The End of Faith. Sam Harris. pp. 78-79