Hitler, Stalin, and Mao Were Not Atheists
Hitler will be first because he’s the easiest. In Mein Kampf and later in a speech at the Reichstag he said, "... I am convinced that I am acting as the agent of our Creator. By fighting off the Jews. I am doing the Lord's work." Oh, but he was just using that for rhetorical purposes, he didn’t actually believe it, right? Wrong, he’s also said, "I am now as before a Catholic and will always remain so." (Both from here.)
Hitler was also a staunch creationist, "Human culture and civilization on this continent are inseparably bound up with the presence of the Aryan. If he dies out or declines, the dark veils of an age without culture will again descend on this globe. The undermining of the existence of human culture by the destruction of its bearer seems in the eyes of a folkish philosophy the most execrable crime. Anyone who dares to lay hands on the highest image of the Lord commits sacrilege against the benevolent Creator of this miracle and contributes to the expulsion from paradise." Any claims that Hitler was inspired by Evolution are mere propaganda attempts to make Evolution look bad.
Besides all of this, it’s quite easy to make a really strong case that, if not for Christianity and its irrational hatred of the Jews up until the middle of the Twentieth Century, the Holocaust wouldn’t have happened. Sam Harris does a wonderful job in Chapter 3 of his book, The End of Faith, and there’s plenty available online, such as here. For some perspective on Nazis, here’s a discussion on books they had banned or burned.
Stalin and Mao are a bit tougher to crack, mainly because there’s no quotations attributable to them either way (at least that I can find, if you know of a reliable source for some, feel free to tell me). But most people seem to assume that because they abolished religion, they must be atheists. In fact, they abolished religion so that they could establish cults of personality, and become gods themselves. They did what they needed to in order to get more power, and religion was a rival power source, which is why they abolished it. Stalin actually reinstated the church after Hitler invaded, because he thought it would help him (from here). Religion simply got in their way, and they eliminated anything and anyone that got in their way.
Even if any of these people were atheists, what would it say about atheists or atheism in general? I’ve never heard an atheist make the claim that because Hitler was a Catholic, Catholicism and Catholics are evil, which is exactly the way people use the Stalin/Hitler/Mao/whoever argument. I would contend that it is far easier to manipulate religion and use it for evil than it is for atheism, mainly because religions are actually organized and unite people with a common belief system, while atheism is unorganized and typically disunited. Religion’s potential for evil is therefore greater, but no more necessary than atheism being evil.
However, put up against each other, religion beats atheism by several orders of magnitude in number of people killed. But that’s another story.