Other examples of nonhuman self-domestication in the wild exist—for instance, the Zanzibar red colobus monkey diverged from the mainland African red colobus in similar ways during its island isolation—but bonobos are the closest and most relevant to us. […] Central to his argument is the idea that cooperative killing of incurably violent individuals played a central role in our self-domestication. Much as the Russian scientists eliminated the fierce fox pups from the breeding pool, our ancestors killed men who were guilty of repeated acts of violence.
Source: wiktionary