To be fair, the things you mention (slavery, segregation, oppression, even genocide) have been justified by people led by pure atheistic humanism effectively cloaking oppression in logic, academia and scientific theory (Mao, Stalin, Hitler, Pol Pot). I think it's fair to say that people with all sorts of ideologies have treated those they fear, don't understand or just plain don't like pretty poorly.

I'll add that some who have been at the forefront of fighting for the abolishment of the oppressions you list have done so while following Christianity (MLK, Wilberforce)