Say that we, primates, have evolved some sort of social structure that values and depends on ‘us’, and antagonises ‘others’.

That would explain that sort of behaviour as well as our human shenanigans (country/religion/“race”/politics/football team/etc).

Perhaps some groups are biased towards ‘us’ (i.e. more accepting), and other groups are biased towards ‘other’ (i.e. more hostile).

The death of a few key individuals can absolutely remove all the commonality between two groups. Seems to have happened with those chimpanzees, and happens all the time in human groups.

It is sad though that this is happening, on top of all the shit that is going on.