It is disturbing to realize that all humans alive today are the direct descendants of the most successful committers of murder, enslavement, rape and genocide, and all historical injustice is just the documented tail end of success and failure in primate evolutionary competition.
The effect of genocides depends more on relative numbers than absolute ones. A few hundred years ago, populations were smaller. Think of the European genocide on the Native Americans, the Maori genocide on the Tangata Whenua people, the An Lushan rebellion, the Neanderthals...