Do materialists really believe in "natural evil", or what do you mean by it? If there is no God or purpose behind things, isn't it a case of "natural indifference" rather than evil? If there is no God, what is the use of a concept such as "natural evil"?
In an atheist perspective why humans perceive evil at all? No other species perceive evil.
There's no evolutionary traces in other species leading to the development of human perception of moral and natural evil.