It is, like everything, double-sided. It can be a basis for a sound lifestyle and a motivation to perform good deeds, or it can be used as an excuse for atrocity, opression and hatred.
I really dislike it when people go on about the horrible, terrible no-good deeds that for some reason, define entire religions and also all of their followers.
The fact is: Most of the time, these people also do not have truely religion-motivated actions, but rather have other reasons for their horrible actions and use religion- or any other cause, really- as a sort of 'excuse'.
And most followers of religion(s) are not evil, or those that would go out on their own and commit atrocities, or people that want mass murder and destruction and war. They are just people, who live their lives- a lot of them, are deep-down, good people.
Atheists are not untarnished, perfect, happy, non-atrocity committing people. They are only about as good as religious people.
Denying that people of a certain group- Christianity, Islam, Atheism, Enviro-nuts, whoever, have not caused atrocities, is downright bad. Because all of these people have caused atrocities of one kind or another.
Was there atheist-motivated hatred and opression in certain places and at certain times in history? Of course. Is it any worse than religiously-motivated hatred and opression? Nope. Because these people were also human, nothing more and nothing less.
Anything can be used as an excuse for hatred and opression. It is just that atheism has historically been as prevalent as other things, to cause this.
Take arguments over "which OS is best". That is a form of it, even if it is extremely mild, but still... it shows the psychology.