Power is a Poison
Solzhenitsyn observes that the poison of power is deadly for those who believe in nothing but themselves.
Power is a poison well known for thousands of years. If only no one were able to acquire material power over others. But to the human being who has faith in some force that holds dominion over all of us, and who is therefore conscious of his own limitations, power is not necessarily fatal. For those, however, who are unaware of any higher sphere, it is deadly poison. For them there is no antidote.
Of course the existence of the religious totalitarian seems to contradict Solzhenitsyn. We can certainly find plenty of religious people who are quite capable of believing that they know what God wants and that they are entitled to act out his wishes.
But even a religious maniac must - logically - recognise he could be wrong. He knows he is subject to another power - even if he has deceived himself that he is its agent.
Those who reject all moral authority outside themselves, the true existentialists, cannot be wrong. This is why the poison of power is so dangerous to them. Not because they are essentially any worse than the religious, but when you see no constraint other than what you are able to achieve with the power that you do have then the temptation to acquire more power, and to protect yourself with that power, can quickly become over-powering.
Logic impels each honest atheist to become a tin-pot dictator. What other choice can he have? Who else can he believe in than himself? When push comes to shove even ordinary standards of integrity and honesty must be sacrificed if they get in the way of success.