It is those very men who most deeply believe that God is watching that are currently committing the atrocities, and in God's very name, no less.
Your level of the basic decency of man is far below that of my own. I believe it is very difficult for individuals to commit attrocities. It is belief systems that are used to manipulated men to commit attrocities. The Romans used one to commit unspeakable acts against early Christians. In this century, you correctly note that Communists have murdered on an enormous scale.
The belief system that goes furthest in restricting atrocious behaviors is one with liberty as the prime engine. That is because any attempt to initiate force against another is a violation of liberty. Liberty neither requires nor precludes belief in God.
Let me cut to the chase here. It is impossible to found a society based on the idea of "liberty" or "fairness" or "justice" or whatever because everyone disagrees about what these ideas entail. Your version of "liberty" is going to be much more preferential to your position than my definition of "liberty". And everyone is going to have to have their own version of "liberty" or "fairness" or whatever.
So, guess what. In a society such as this someone is going to have to decide which version of "liberty" or "fairness" or "justice" is going to prevail. And because you can't appeal to God for authority, well then you have to appeal to force. If I refuse to go along with a version of "liberty" that I personally disagree with, then you're going to have to "initiate force" against me to comply, in direct contravention of your stated "gold standard" for libertarianism.
And who gets to decide what "liberty" or "justice" or whatever actually means? An intellectual elite? The people with the most guns? Who?
From my point of view, libertarianism--the atheistic variety--looks like this: an abstract idea that requires some sort of elite to make arbitrary decisions to stop that abstract idea from plunging into absurdity. These arbitrary decisions are backed up with force.
Gee, sounds like tyranny to me.