This is from Richard Wurmbrand's Tortured for Christ.
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
There are moral pits that Christians can fall into. Here is a Chesterton Father Brown story that describes one. But there is the "everything is permitted" moral pit that requires atheism. And it is a deep one. It is far from the case that atheists must fall into it. But I think it delusional to deny its existence, or to deny that it was partly responsible for the horrors that took place behind the iron curtain.