Hitler, Mao and Stalin didn't kill people because they didn't believe in God; they killed people because they had ideologies that diminished the value of the individual Human Being and aggrandized the collective - society and the state that repesents it in the case of the communists and race in the case of the Nazis. To such ideologies, abstract ideas take precedence over people and their survival as individuals. Most of the wars throughout Human history were either caused or endorsed by religion in one way or the other. People have been killing each other in the Middle East for thousands of years over religion.
You just contradicted yourself, with this statement. "Aggrandizing the collective" is precisely the point. Therefore, whoever runs the collective is the top dog. Plus, in order to establish this collective, what is the ONE thing they all had to eliminate: FAITH IN GOD. That's why Hitler proclaimed that to establish his collective, Christianity had to be stamped out "root and branch".
They not only didn't believe in God, they wanted to supplant Him as the object of worship among their people. Hitler wanted people to look to the swatiska, not the cross, for their salvation.
The kings of medieval Europe invoked God before going into battle and slaugtering thousands of people. The emperors in ancient Rome made offers to Mars, the god of war, asking him to vanquish all their enemies and rape all their women. People have killed in the name of God or gods throughout history, but no one has ever killed because they don't believe in God. It was a mere coincidence that the dictators of the 20th century who killed millions were atheists. They could have been very well like Richard the Lionhearted, who slaughtered a million Arabs, including women and children, in the name of God. If they were monsters because they were atheists, then they should be monsters for evryone equally, but they weren't. Hitler, for instance, valued the lives of Aryans immensely, and even in 1944 when Germany was on the brink of defeat and food was scarce, he never ceased from giving German children milk at schools. He despised only the Human lives of certain categories of Humans, and he did it because of ideology and not because of atheism. If it were because of atheism, then he should have despised the lives of all Humans equally and not only non-Aryans, because if you are only moral if they is a God and you believe there is none, then you should be equally immoral towards everyone.
SUCKMYMUSCLE
Hitler was gracious towards Aryans, because (as mentioned earlier) he wanted people to look to HIM as their salvation. Had they not adopted HIS values and his ideology, Hitler would have destroyed them (Aryan or not) just the same.
The ideology here that he, Mao, and Stalin employed was that there is no God and that their government was the "deity" to be worshipped. That is why I've repeatedly stated that atheism, in practice, is simply man worshipping himself.
As for the war stuff, again, the Crusades can't even come close to racking up the bodies that Hitler or Mao or Stalin amassed during their regimes.