I know how many those people killed. I don't know why you think I don't.
The absolute numbers may well favor the atheists. But there's only one reason for that: those are who the genocidal maniacs happened to be in the 20th century. The next genocidal maniac will probably blow all of those away, regardless of religious beliefs, simply because of the number of people available to kill, and the technology available with which to kill them.
The world population at the time of the Great Leap Forward was 2.4x larger than at the time of the Taiping Rebellion. It was about 6x larger than at the time of the French Wars of Religion and the early phases of the conquest of the Americas, 10x larger than when the Crusades got started, and 15x larger than when the An Lushan Rebellion killed perhaps 5% of all humans on the planet.
It would have been difficult to kill 100 million people during the conquest of the Americas, because that probably would have required millions of immigrants just to bring the population back up to zero afterwards.
In terms of languages lost, cultures destroyed, peoples wiped out, land permanently lost to conquerers, and duration, the Americas "win" by a huge margin.
By the way, do you know why the Atlantic slave trade got started? It's not because Africans made such good slaves. It's because the conquerers of the Americas kept working their native slaves to death and began to run out of natives.
China's population recovered to the level prior to the Great Leap Forward in a few years. I'm not sure of the USSR's population ever significantly dropped, as it was a sort of slow burn that population growth kept up with. Europe's population recovered to pre-WWII levels in a few years. How long did it take the Native American population to recover to pre-contact levels? It's hard to tell, because the pre-contact civilizations were so devastated that there's no good idea of how many people there were, but it's likely that they still haven't recovered, and may never do so.