Why Body Counts Can't Tell Us the Motive
The theist's strongest card is the sheer number of dead under communist regimes. This post argues that the size of a death toll mostly reflects the method and means of killing, not the motive behind it.
Motive and method get mixed up
Motive matters. A strong enough motive can push the numbers up. But the method of killing plays a much larger part in how many die, and the two get tangled together. So you can't read motive straight off a body count.
Technology changed the scale
The 20th century was the first time states could control vast territories with railways, telegraphs, radio, censuses, secret police and industrial weapons. Huge countries could be unified and armies coordinated across thousands of kilometres. That made killing on a new scale possible, and explains why the same century produced two world wars and the atomic bomb. Nazi Germany, which was not atheist, killed on the same industrial scale as the communist states.
Even deaths as a share of population don't fix this, because technology also lets a state reach a larger share of its people. Comparing the 20th century with earlier ones by body count tells us about technology, not belief.
The rate of killing
The Mongol conquests are often raised as a pre-modern counterexample. But look at the pace. The Sino-Japanese War of 1937 to 1945 killed around 20 million people in eight years, about 2.5 million a year. The Mongol conquests, by common but very uncertain estimates, killed tens of millions over well over a century, a few hundred thousand a year. Modern war killed several times faster.
Such counterexamples are also few, and the Taiping Rebellion already belongs to the 19th century and a far more populous China.
Famine is its own category
Famine can kill vast numbers even without modern technology, simply by cutting people off from food and seed. It skews any comparison and must be treated separately.
It is also where most communist deaths came from, through collectivization and policies like the Great Leap Forward. Many of those deaths were the product of disastrous policy rather than deliberate killing, which takes them even further from any motive, atheist or otherwise. The same rule applies both ways: famine deaths in religious wars shouldn't inflate religion's tally either.
Conclusion
Body counts measure power and method. Once technology and famine are accounted for, the numbers no longer support the claim that atheism is uniquely deadly. The question of motive has to be settled on other evidence, and that evidence points to ideology, as I argued in Ideology, Not Atheism.