Sohlznetsin claims over a hundred million were slaughtered by Stalin alone, ditto for Mao......how's them numbers compare?
Read any history of 30 years war- start with the Encyclopedia Britannica, and go from there - they would have a bibliography. And as has been noticed to you on this thread before, in religion wars the fight was - expressly - for the sake of, and in the name of, a religion [whatever religion]. Human sacrifice was again done in the name of a religion. Causation there is self-evident and self-proclaimed. The wars and crimes of atheist dictators were never done for the sake of, or in the name of, atheism, but rather for much more mundane purposes [personal aggrandizement, power struggle, territorial conquests, racial theories, class struggle, enemies of the people, collectivizations, great leap forward, or rather sideways, and so on]. thus you would have a hell of the time trying to establish causation with atheism.