I saw a documentary (animal Planet or Discovery) about gang like chimp behaviour.
They would patrol their terrytory and the documentary crew managed to film a brutal attack to an invading one. The gang attacked him until the invader was dead. Then, they walked away and left the corpse.
They are exibiting NATURAL "chimpanzee" behavior...not mimicking humans. Chimps are naturally nasty creatures all on thier own.....not because of anything WE did/do.
Gorillas on the other hand......aren't generally prone to such behavior. They're much less aggressive and nasty.....at least, they haven't been "caught" doing such things as yet.