They don't believe, and nor do they disbelieve. You should learn what being an atheist actually means, this discussion might actually be productive.
That's getting a bit snotty.
Atheists don't believe in god{s}.
Babies don't believe in god{s}, until someone fills their heads with lies.
So you count a guy with a very unconventional religious beliefs, who in semi-private despised Christianity, try to invoke the 'no troo Scotsman' fallacy, (and indeed no Christian has ever killed in harmony with the teachings of Christ. As you and many others have pointed out, killing is not against atheism. Atheism's only tenet is of unbelief in God(s). Therefore, mass murder cannot be against the tenets of atheism) yet try to pretend away the mass butchery by atheists, in order to promote atheism? And its not only the 20th century. Perhaps you might like to learn about the French Revolution as well. You may as well say black is white. Perhaps you have a 'no God' delusion
OK, so you played the "no true Scotsman" fallacy. Good for you.
You confuse murder IN THE NAME OF vs MURDER BY.
See, you are being a hypocrite here. If an atheist does something wrong, you blame atheism. If a believer does something wrong, you blame the individual or whatever else you can find to keep it from being viewed in anyway related to religion.
Maybe you should read your bible for once. There's some pretty nasty stuff in there, unless you think god sending a bear to kill a bunch of kids for laughing at a man for being bald. I guess you like slavery too, since that's a good thing in the bible. What about joyfully throwing babies down the rocks below? What about slaughtering entire villages, except the virgin girls who are then used as sex slaves? Yeah, this is where you try to throw in massive amounts of apologetics to justify and distance yourself from what the bible says.