It shows how little you know about Stalinism. Stalin, Mao and Pol Pot all persecuted people for their religious views precisely because they were atheist and there is no God. To believe that there is a God, was to contradict their belief system. It was all done very much in the name of atheism. Obviously there are few such examples because atheist states and governments are extraordinarily rare.
You can virtually guarantee this one cropping up in religious debate, a sort of accusation if you like that they committed such horrendous crimes because they were atheists which is blatently false. Stalin commited crimes based on his version of Marxism, Hilter based on eugenics. Added to which it's probable that Hitler was in fact a Catholic.
Individuals sometimes do dreadful things but no-one goes to war on their belief that there isn't a God. On the other hand, depressingly frequently, people go to war over (or on the pretext of) various ancient bit of script.