I thought it was obvious that when I referred to Communist massacres in the Twentieth Century, I was referring to Stalin, Mao, Pol Pot and other Communist dictators. Accounts of the various mass deaths they were responsible for are incredibly widely available and can be found with any search engine; this is essentially like asking me for "citations" for the Holocaust.
(Please do not try repeating at me that Communism and Atheism are not interchangeable, I already know that. The fact remains that Communism is a relevant example of the susceptibility of atheistic ideologies to the same kinds of bloody-mindedness that have periodically affected religion.)
I will repeat it, because you're wrong; Atheism is not the same as Communism. Nothing about being an Atheist tells me I should believe in the ideals of Communism or Objectivism or being a Relian. Just because the Relians are Atheist does not mean that I, as an Atheist, believe in advanced space reptiles. Just like being an Atheist does not mean I share my Atheist friend's opinion on abortion. The only trait I share with other Atheists, the only trait that Atheism proports, is that we do not believe in a god, be it Thor, Vishna or Yahweh.
As I've pointed out, Stalin and Mao both cultivated huge cults about themselves as semi-divine figures. If you want to claim someone was motivated by their Atheism to kill someone, you must give a reason why. If you can find any source of proof which shows that Mao or Stalin's lack of belief in God was their reason to kill, I will be glad to agree with you. But currently, your arguement is 'Mao was a Trekkie; this was obviously the reason he killed thousands' without giving a source where Mao is quoted saying that "I will murder all people who thought Shatner was better; Piccard is the obvious choice".
Edit: If you want citations for Hitler's motivations, there are plenty available. Go listen to his speeches, or read Mein Kampf.
Edit #2: On re-reading, just realized I completely misread your first line; apologies, it's pretty late here. However, the case in point is still that Communism does not say anything about someone's Atheism. Stalin and Mao were Atheists. However, unless you can give me a source where Mao or Stalin argued their deeds or showed evidence that their deeds were done because they believed 'there is no Thor', then their Atheism is irrelevant; it has as much impact on their motivation to kill as their preference in book genre or favourite cheese has.
There are times when the lack of a belief in the existence of a higher power can lead a person to devalue human life. If nothing is sacred human life cannot be sacred so the taking of a life is no big deal. They don't have to be an avowed atheist. They only need to think like they've been told an atheist thinks.
This is based on the assumption that Atheists do not value human life, which is not something inherant to Atheism. Theists and Atheists alike do this; some religions devalue the the life of homosexuals and women as lower than those of straights or other men. Some Atheists devalue the life of people based on race. These things can be inherant to religion (Remember, Christianity does -not- promote that human life is sacred; it promotes that humans are sinful, imperfect beings who deserve to suffer and our only worth in life is to enter the glory of God's grace through subserviance and blind faith), but are not inherant to Atheism (which does not make any comment on how 'sacred' human life is; the only thing Atheism comments on is whether a person does or does not have a belief in a divine being).