If there is no God, there can be no such thing as evil. This, at least, is the opinion of Jean-Paul Sartre. He lambasted the French atheistic professors who denied the existence of God yet wanted to hold on to a system of morality. You cannot have it both ways, he argued. Without God, there is no human nature, no order, no basis for anything.
Man is abandoned and alone.
Despite the fact that he later abandoned his existentialist approach to the question Sartre's critique remains valid. If there is no God, there is no basis for distinguishing between good and evil. If humanity is simply the accidental by-product of a blind evolutionary drive, then humanity has nothing to complain about when the cosmos continues on its blind evolutionary path. It may be inconvenient, unpleasant, even horrible. But it is not good or evil.