Stalin and Mao found no ideas in atheism -- lack of belief in a god or gods -- that led them to kill anyone. This simply because there are no such ideas. Atheism has no dogma, no canon, no nothing. The state of atheism consists of a lack of belief in a god or gods, and nothing else. Consequently, ideas like "kill some number of people" by definition come from another source. And in particular:
Stalin and Mao were psychopaths (crackpots, frankly), and that is where you want to look to find out what drove them to kill. Whatever you find, it is an absolute certainty it won't be atheism.
However, the crusades were, in fact, driven to a significant extent specifically by theist reasoning, canon and dogma. As were the murders and tortures perpetrated during the inquisitions, the witch-hunts and subsequent burnings, blood libel, and pogroms, many events such as the 9/11 incidents, various wars, as well as the lesser but still despicable centuries of subjugation of women, repression of sexuality, interference with relationships and legislation, social ostracism, and so on.
I will also say that theist thought has also been the prime motivator for a massive amount of great art in many forms -- sculpture, paintings, architecture, music and a whole host of various other artifacts, and when charity and compassion are foremost and the compulsion to impose belief is absent or at least minimal, theism is at its absolute best at doing little to no harm while doing extensive good. This does not, in any way, say that we should forget, or forgive, or ignore, the many evils done in the past, being done now, and those impending, in the cause of theism.
So you want to be very careful before you go waving Stalin and Mao around as examples of atheism causing problems, or, as a counter to the historical fact of the murders committed directly for the (various) causes of religion . Atheism providing a rationale to harm others is not the reality. It's never been the reality. Claiming it is the reality is either disingenuous or ignorant.