Spend long enough in the company of less educated believers, and should the subject turn to human morality, sooner or later you will be told that Hitler, Stalin, and Mao were atheists whom were responsible for the deaths of over one hundred million people during the last century. Ergo, Atheism is route one for anybody partial to a little ethnic cleansing, genocide, or general disregard for human wellbeing. I'm going to spend a brief time putting each of these suggestions to bed; it won't take long because anybody who makes these claims just hasn't done the hard yards required to confirm the data.
Adolf Hitler was a grotesque creature whom rose to prominence on the wave of vast amounts of German anti semitism dating back to the late 19th century. If you're looking for clues as to what might have prompted a mindset capable of the holocaust you'd do well to start with the ill feeling and general suspicion Germans had towards the Jews. As for Hitler himself, in his biography Mein Kampf he specifically ventured that Jesus cleansing of the temple was inspirational for him, and the driving out of the Jewish money lenders was pivotal in his understanding of how they should be dealt with. I could add that on the belt buckle of every Nazi soldier was the phrase "God With Us" and it becomes clear that Hitlers atheism was seriously over stated. He refers to Christianity over two hundred times in his writings, which should give you some indication as to his allegiance. However, in his later years he developed a mistrust and dislike of organized religion and as such I would not wish to describe him as a Christian in the conventional sense.
Stalin was, on the other hand most definitely an atheist, although he did train for the Russian seminary so he knew something of the clergy. Yet one has to understand something of the communist regime before one can label him so quickly. Communism was about collectivism, about the good of the group taking precedence over the rights of the individual. It was a totalitarian regime with allegiance demanded to an absolute ruler, and a failed human experiment. It had nothing specific to do with Atheism, and Stalin never committed his atrocities in the name of it. One might argue that the absence of belief frees a person to do as they want without fear of reprisal, but ask yourself, does that make the claim for God any stronger? It's not an either/or question; problems of morality and social justice are vastly complex and many bright people agonize over just how we can live a good life.
Finally Mao, and in truth he falls into the same category of Stalin; namely an individual who set himself up as an absolute ruler and demanded total allegiance. Again, I'm unaware of any published quotes from this evil man that expressly state he carried out his tyranny in the name of unbelief?
Now that was a very brief insight into three of the most profane human beings ever to walk the Earth. Thousands of words have been penned on every known and conjectured detail in their lives. What forged these monsters is probably going to be a variety of factors, but in every case we can be sure of the following; even if they did what they did because they had no fear of heavenly punishment this would in no way enhance the claims of religion. Every single atrocity carried out during the entire course of human history couldn't do that. Now granted, belief in an imaginary authority figure may in some cases improve behavior, but there's equal evidence that it can also turn it for the worse. How many slaves kept? How many witches burnt? How many crusades embarked upon? And how many have silently suffered under the yoke of false guilt and repression all because of belief in a phantom?
I'm 40 years old now, and have lived life as both a committed Christian and also as a staunch secularist. The latter has enabled me to live with much greater honesty, acknowledging how imperfect I am and how complicated the world can be. I know that for many issues there are no pat answers, no easy solutions to be secured. Should we expect it to be otherwise?