A rough head estimation from WW2 (80 megadeaths) by Deist/Atheist Hitler, Mao's Great Leap Forward (25 million), Chinese Civil war (30 million) and the Holodomor (5 million). Estimates vary wildly of course, but I think you'll find that the post 1700 world (when the enlightenment made Atheism more acceptable) those without religion have been responsible for far more death than those with in absolute terms. Far worse in proportional terms.