Here's a study on middle ages Croatia where 20 percent of skeletons showed cranial fractures (which doesn't even include murder by flesh wounds):
https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.22083
Here's a study on London cemetaries which found a 7 percent rate (again this excludes murder by other methods):
https://onlinelibrary.wiley.com/doi/full/10.1002/ajpa.23288
"e.g. famine or other loss of resources, usually man-made by outsiders or despotic rulers"
Demonstrably wrong in the case of the Waorani. It was a blood feud going back generations, long before any contact with other societies. So yes, a Leviathan would've stopped it.
https://www.nature.com/news/2009/090511/full/news.2009.463.h...
So culturally preserved hatred (which became an outlier over a period of time where other cultures preserved less hatred), not much different than the Hatfields & McCoys.
Past research suggesting this kind of thing -- like the single study of a single tribe that you are giving us with your argument -- has been debunked by more nuanced, modern (and less colonial/racist) research. There are far more peaceful societies known to us than violent ones, though unfortunately we are mostly forced to participate in one of the latter.