If you were to only examine the big three Atheistic regimes of the 20th century—Mao in china, Stalin in Russia, and Hitler in Nazi Germany—then you would discover that they are responsible for more than 100 million deaths (and that does not even include others like Pol Pot’s mass killings in Cambodia). Dinesh D’Souza observes that:
"Religion-inspired killing simply cannot compete with the murders perpetrated by atheist regimes. I recognize that population levels were much lower in the past, and that it’s much easier to kill people today with sophisticated weapons than it was in pervious centuries to kill with swords and arrows. Even taking higher populations into account, atheist violence surpasses religious violence by staggering proportions. Here is a rough calculation. The world’s population rose from around 500 million in 1450 A.D. to 2.5 billion in 1950, a fivefold increase. Taken together, the Crusades, the Inquisition, and the witch burnings killed approximately 200,000 people. Adjusting for the increase in population, that’s the equivalent of one million deaths today. Even so, these deaths caused by Christian rulers over a five-hundred-year period amount to only 1 percent of the deaths caused by Stalin, Hitler, and Mao in the space of a few decades."
D’Souza further adds that, “If Christianity has to answer for Torquemada [cf. Inquisition], atheism has to answer for Stalin. By the same token, if the ordinary Christian who has never burned anyone at the stake must bear some responsibility for what other self-styled Christians have done on behalf of religion, then atheists who think of themselves as the kinder, gentler type do not get to absolve themselves for the horrible suffering that their beliefs have caused in recent history.” All loss of life is tragic, and I am certainly not trying to “white-wash” the evils done in the name of Christianity, but the facts of history show that atheism, not Christianity, is responsible for the mass murders of history.
For more on interacting with the New Atheism, check out Dinesh D'Souza's book, What's So Great About Christianity