You may have heard that the greatest evils in the world were perpetrated by communists who didn't believe in God. Let's disprove that claim.
The reason communist societies love to flirt with atheism is this: coopting atheism is useful to communism inasmuch communism seeks to establish itself as the sole authority on all things public -- which is impossible if you have the competition (religion) doing it. Really, all communism does is replace one form of authoritarianism for another.
Let's start with the obvious: it is possible to not believe in God, and not be a communist at the same time. The vast majority of atheists are sensible, reasonable people who don't believe in communism and murder but in humanism -- free thought, which is diametrically opposed to communist regimes. Ergo: the idea that atheism begets communism and murder is a myth.
Now let's move on to our second argument: the three big murderous communist societies (Mao, Stalin, The Great Leader) have sought to -- and ended up -- establishing their leadership as the supreme authority -- not unlike the Pope. There's no atheist teaching that "thou shalt murder". Stalin didn't just murder priests -- he was an equal opportunity murder, bent on destroying not just religion but anything that threatened his bloody rule. Why do you think Stalin murdered people? Because he was a paranoid lunatic who thought everyone was out to get him, not because atheism commanded him to do so. And Mao? Oh, well, he just starved twenty million to death for a purely political matter -- his "great revolution". Atheism there? Not at all.
(I'm not citing Hitler because he was a devout Catholic and he wasn't communist. Oh, did I mention that his rule took six million lives?)
However, there's a rather solid case to be made against religion, because all three religions have explicits "thou shalt murder"s. The three big ones preach violence and blind obedience (authoritarianism) directly. All I have to do to prove my point is point to the Bible and the Koran, then nudge you to Muslims wanting to murder Jews, Jews wanting to murder Muslims (both over "Holy Land"), the absent Twin Towers, and American Christians rah-raing against a million dead Iraqis and claiming a victory for God (google Project for a New American Century). Oh, and Hitler -- Gott mit uns, remember?.
In summary: The greatest murderers of our age didn't kill because atheism commanded them to kill. However, the amount of religious-generated hatred, violence and murder is directly a consequence of religious teachings, and justified with it.