Everyone always thinks Hitler was an atheist for some reason...probably because they want to paint atheists with the broad brush of evil. Well I would say religion has done way more evil crap than all the world's atheists combined. But that's all I want to say about that.
Hitler was an atheist, at least in the sense that he was most certainly not a Christian and there was plenty of evidence that he wished to purge Germany of all traces of Christianity.
Stalin was an atheist.
Mao was an atheist.
Just between those 3 your'e talking over 100 million deaths.
Now, I'm not religious. I'm agnostic -- that is, I don't believe in God but I also won't say there isn't one as I just don't know. Humans seem to be hard wired to believe in *something*. Not all humans of course but sufficient numbers that in the absence of traditional religion, they'll invent one and that often comes in the form of an authoritarian state.
Very often, it seems, you scratch an atheist and a statist bleeds. And the problem with statists is that they want to use the government to enforce their world views (whether that be enviromentalism, health food, etc.).
I don't mind someone telling me it's a sin to buy a 32 oz soda until they make it an actual crime to do so. 