This is quoted in William Lane Craig's "The Indispensability of Theological Meta-Ethical Foundations for Morality," to which I link.
The cruelty of atheism is hard to believe when man has no faith in the reward of good or the punishment of evil. There is no reason to be human. There is no restraint from the depths of evil which is in man. The Communist torturers often said, 'There is no God, no hereafter, no punishment for evil. We can do what we wish.' I have heard one torturer even say, 'I thank God, in whom I don't believe, that I have lived to this hour when I can express all the evil in my heart.' He expressed it in unbelievable brutality and torture inflected on prisoners.8
Admittedly, persons with that kind of power over others often mistreat them, because they can. But does atheism, or just the failure to recognize God, remove a barrier to human depravity for those who are put in that kind of a power situation.
I realize that we don't have a controlled experiment here.