No, he was not an atheist, so you may take the holocaust and WW2 casualties of your list, which, I note, stands entirely unsubstanciated, even less credible after this major blunder.

Hard to tell whether Hitler really believed or used Christianity, as with many leaders.

I'd venture to say that it was not "the atheists", but the Christians who have the worst record of massmurder, if one considers a concerted, organised effort over a period of almost 2000 years of torture and slayings, much of it killing each other in the name of their god.

Barbaric - isn't this what you enjoy so much accusing Islam of? - 2 versions of the same god, it seems.