We can’t have this discussion until we agree upon some things.
Does Buddhism count as atheism? It is a belief system. There are probably more Buddhists in China (official statistics disagree) than there are people in the U.S. How do we rate them? A lot is at stake; perhaps fifty million people died during the Cultural Revolution and the Great Leap Forward. And if we count religious belief in Japan, we have Imperial Japan’s occupation of China in 1945.
Were the Crusades and the Spanish Inquisition inherently moral? Were the people doing God’s will, and therefore their mass murders were acceptable to God, and therefore don’t count? In modern figures, the death tolls may not be impressive, but as a percentage of the known world at the time, they were pretty bloody ambitious wars.
What about war in general? World War II had the greatest number of deaths of any war in history we know of. Does it magically “not count” because we had God on our side? (I note that the U.S. Army practically requires religion in the trenches; read up about the non-promotion of atheist non-coms.)
What about the Ancient Romans and Ancient Greeks? They believed in gods, it’s true, just not the God, and yet they invented the legion, the phalanx, and other instruments of mass death. The Romans enslaved and crucified people, and yet they were religious.
What about slavery in general? The U.S., a very religious country, was among the last to abolish it, in a deeply religious South. Moral or not?
What about epidemic? Do we blame the highly religious Columbus for the death of possibly 20 million Native Americans, because he brought epidemics from Europe? Or does he get a pass because it was an accident?
How about the bombing of Nagasaki? I bet that pilot was religious.
Irish Potato Famine, anyone? Or even British imperialism in general: the Boer Wars, the occupation of India, and so on.
Human sacrifice practiced by the god-fearing Aztecs?
What about the Catholic Church burning scientists at the stake such as Giordano Bruno? I suppose that was moral, too, because the Church ordered it. Same goes for the Salem Witchcraft Trials; doesn’t count because their hearts were in the right place. Can’t blame 'em for killing innocents; they were aiming for Satan and missed.
And if we’re going to count the Holocaust because an atheist ordered it, what about all the massive amounts of manpower it took to make it happen? I suppose they must all have been atheists too, and you can prove it, right? And if they were Christians, they get a free pass because they were only following orders.
Yeah, if you simply want to look at death and oppression, religious people don’t have a glowing record of purity either.