In a naturalistic world in which there is nothing supernatural, nothing other than the material world, and everything there is can be summed up by what we can touch, see, hear, feel and measure, survival of the fittest reigns. In a world like that, what is wrong with genocide?
Genocide is like the ultimate survival of the fittest. The superior people group dominates, overcomes and wipes out the inferior people group. What could be more Darwinian? What could be more natural in a naturalistic world?
This, in fact, is largely the history of the world. Why, then, is this expression of survival of the fittest wrong?
Thankfully most people today recoil from such a notion, but on what basis?
Theists claim that people could not do good without God. But, people do good things all the time, even without believing in God. Atheists, agnostics and people of all stripes can do good things and they do good things.
Obviously, believing in God is not a prerequisite to doing good things. A better question, is whether good can exist without God?