Good points, but it any case, it's true that chimps in general treat group members very differently to outsiders isn't it? Those behaviours that de Waal mentions seem probably directed towards group members. Are there any documented chimp populations where chimps aren't violently aggressive towards members of other groups?

I remember reading, not sure if it's from de Waal, about chimp "raiding parties", where groups of young males will get excited and loudly vocalise as they gather together and head towards a neighboring territory, but when they get close they all go very silent, and will attack individuals from a neighboring troop if they sufficiently outnumber them. They tend to target the face and genitals when attacking other chimps, a different behaviour to when they're hunting monkeys, for example. I think Wrangham mentions that some chimps will hold the targeted individuals' limbs while others attack.

Aside from the brutality, these behaviours seem too cogently goal-directed and sophisticated to just be responses to environmental pressures. There's some deeper reasons involved, imo, even if the severity of the violence is exacerbated by resource and territorial pressures.