The primatologist Richard Wrangam once advanced the theory that tribe vs. tribe conspecific homicides - what he called coalitionary killing - are an evolved trait that was selected for in primates by some kind of pro-homicide selection pressures in the ancestral environment (where homicide reliably grants an advantage to the expected relative gene frequency of the perpetrator's genes).

I haven't kept up with biology for years and don't know what the current consensus on the topic is but it's interesting to consider if some environments naturally promote the unlucky inhabitants to harm each other.