It's absolutely ridiculous to blame atheism for say stallinism. It's as reasonable as saying that Stalin's moustache is somehow to blame as well.
If you claim atheism is cause of stallinism you should be able to quote a passage from holybook of atheism which commands people to commit genocide.Or maybe you should be able to quote a founder of atheistic move? You can't do it. There ain't any holy book of atheism. You may quote some book written by atheist but it would no better than quoting Main Kampf as an evidence of painters being evil. Also there is no founder of atheism since it's default position.
Atheism is just one conviction that is there is no reason to believe god exists, or to be more correct it's implausible god exists. That's everything. It's not a packet of beliefs, it's not a whole system. It's just one single belief.
You may say "oh, it's quite convenient to not believe in god, you don;t have to worry about eternal punishment" and it's obviously dumb respond. Why? Because not only it can be easily reversed but what's more important it totally misses the point. First of all it may be indeed convenient to get rid of concerns about your eternity BUT I think it's much more convenient to believe that not only you're not going to be punished in the afterlife, but you'll be even rewarded since god must be on your side. This is why saying that atheism is convenient is shooting oneself in one's foot. But there is my second objection that is this statement is off the point. The issue is: Does atheism induces crimes? And again to me the answer is obviously not. There are no atheistic authorities, there are no atheistic rituals, there are no atheistic codes or holy books. Obviously there may be people who are atheists and who try to convey how people should live according to their worldview, but again when you're atheist you're not obliged to conform to anything other atheists think and yet still be atheist.
Obviously in case of religion it's all different. You can't call yourself christian and yet say "Well I don't really care what that Jesus guy wanted me to do" or "I'm catholic, but who cares about pope" or "Well I'm muslim but I really don't like prophet Muhammad"
That's what theists extremaly often don't understand. They think that since their whole worldview may be basically described using just one word like christian or muslim it necessarily means that the same applies to atheism. Bullshit. You can't tell anything more about atheist that he simply doesn;t believe in god and/or think god is improbable. Whereas when you call yourself christian I can easily tell what's your standpoint on homosexuals, abortion, freedom of speech with respect to your god etc. Or at least I should be able assuming you really believe what you believe. And I can do this using the book that is the basis for your religion. You can say that since I'm atheist I'm more likely to say support abortion. Statistically it's probably true, but again it's not about statisticall truths but rather whether you can say that my standpoint on this issue is predefined by my atheism and the answer is obviously not. You can be atheist and oppose abortion. The same goes for literally everything. And that's all because atheism is just one independent belief.