Let's see. You could ask atheist Alfred Kinsey or any of the many children he exploited for sexual purposes. Or, you could ask Than Shwe, devoted atheist and dictator of Burma. Or you could ask the millions of children he exploits or any of the poor class of his country. What about the militant atheist Kim Jong Il? You could ask him what role atheism played in his decision to kill around 5 million of his fellow Koreans. Want someone closer to home? Try Jeffrey Dahmer, the atheist who killed quite a number of men and boys, dismembered them - and then ate them. He has repeatedly said that he feels no remorse or accountability because there is no God to account to. The irony is that if he's right - if there is no God - he's also right that there is no accountability, no morality, no reason NOT to kill or eat your victims.
You could ask Benito Mussolini if he weren't already dead. Likewise for Mao Zedong, who repressed religion, burned religious books, etc and was directly responsible for immense national poverty, the persecution of over 36 million Chinese and the deaths of at least 5 million. There were also a number of reports of cannibalism.
Pol Pot & Joseph Stalin were both devoted atheists. Both of them were also responsible for the mass murder of millions. Pol Pot was unique in that he was an atheist as well as creative. Few atheists are, but he realized that every time you murder someone by shooting them, you have to purchase a bullet. So he resorted to using axes, hammers or sharpened bamboo sticks to kill his victims with. Stalin just allowed his people to starve and purged his state of anyone he deemed to be an enemy. He was directly responsible for the deaths of over 10 million people, including over 100,000 Catholic Priests and Nuns.
The question of Hitler is debateable. He was raised as a Catholic but later seemed to reject and even loathe Catholicism/Christianity. He too was responsible for the deaths of millions, including anyone who he could find that happened to be Jewish or who dared speak out about Naziism. What is NOT in question was the fact that he became deeply involved in the occult, which from the perspective of Christians, is a close parallel to atheism.
While it's true that horrible acts such as the Inquisition and the Crusades killed many and were committed by self-proclaimed Christians, the difference is that those people had to COMPLETELY IGNORE what Christ really taught about loving your enemies, etc, in order to commit those acts. Christ warned of people like that, saying that there would be many who would claim to do works in His name. But to many of those, he would answer: "I never knew you: depart from me, ye that work iniquity." (Matt 7:23) They were never true Christians.
Meanwhile, atheism is the rejection of God, morals, principles, absolutes, consequences and so on. So all of those individuals I identified above are ABSOLUTELY a core part of historical and contemporary atheism. This is a very silly question.