Really?
Scientific materialism was the base philosophy of the Soviets. In other words, men are nothing more than an agglomeration of nerve ganglia, muscle, bone and whatnot and nothing more.
The Christian brief is that men are material and soul. I cannot kill you because I did not create you; and, we are ontologically equal before God. Remove the soul and no men are equal; our physical differences define us.
If you firmly believe that men are nothing more than animals and that God is not watching, it's not really all that difficult to commit all kinds of atrocities.
LOL -- Lysenko was about as "scientific" as Miss Cleo.
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.