The Evolution of Genocide

In 1994 in a period of 100 days the Hutu in Rwanda killed 800,000 Tutsi and this was accomplished mostly with Machietis. In 1915 there was a Systematic killing of the Armenian population by the Ottoman Empire. The question is what is going on here? We see this kind of thing over and over in history. How can we understand this characteristic of human behavior? Because it's of deep importance for us to understand this if we want to have any hope of preventing this in the future.