Easy enough to guess how that works.
Give chimps/gorillas/humans enough territory and they're peaceful, make them compete with others of the same species for limited resources and they'll turn violent.
I don't need much more than basic evolutionary theory to support that.
Gorillas are largely vegetarian, and generally peaceful. Humans are much better example of “violent tribal monsters” than gorillas.
Cf. https://www.washingtonpost.com/archive/opinions/1977/12/25/t...