Let’s put this one to bed shall we?
Yes, atheists have killed millions, so have theists. There is one big, important, powerful difference between the two though.
Atheism itself has never motivated, excused or caused any deaths. Atheism is just the personal statement that one does not believe in god or gods. How, exactly, can that – in and of itself – motivate anyone to kill, exterminate or destroy? It can’t.
When people talk about ‘atheists murdering millions’ what they’re actually talking about is Communism* (Fascists were conspicuously religious and the link between anti-semitism and religion is pretty fucking obvious).
Communism in these states and instances, Mao, Stalin, Pol Pot etc became a religion. Ideology can become a matter of faith as well (as we’re seeing with capitalism around the world this decade). More accurately, Great Leader became a religion and their paranoia more than anything lead them to crack down on anyone and anything that threatened their position, which included religious organisations, if they couldn’t suborn them.
Religion on the other hand? Well, religion can easily excuse or motivate genocide. Just ask William Lane Craig.
*Yes, I know these states weren’t Communist either, but I thought I’d stick to correcting one stupidity at a time.