Any piece of fiction should be considered on its own for whether its characters are plausible and consistent.
But it's the exception not the norm in human behaviour.
Remove all of this.. what happens.. humans will eventually go back to primal instincts
Humans are social creatures, but not social with the entire world.
The modern view is that most likely hunter-gatherer societies, which didn't have fixed lands and had no general reason to be territorial, had little warfare. It is only with agricultural societies, where competition for land becomes a factor, that we start finding evidence of warfare, as far as I understand.
What's interesting it's that researchers that lived with the Yanomami and other Amazonian peoples noted that most inter tribal conflicts weren't about competition for food sources-that was rather plenty- but instead for women. In polygamous societies, high status males acquired many wives and young males frequently kidnapped neighboring tribe's women which sparked constant retaliatory raids.
They weren't run by man children.
My parents lost members of their families to genocide. The people who planned it were not stupid. Ruthless yes, calculating and callous and cruel yes. But not stupid, and it's both amusing and highly frustrating to see people who've mostly never had to confront these things pretend that it's just the work of "man-children" (because gods forbid that we contend with the idea that the intellectualism and intelligence we place on pedestals might ever be morally bankrupt).
It's interesting though that you are nitpicking about "distasteful" vs "morally abhorrent" (and will probably do the same about "unpleasant") while calling my point pedantry.
I'm comfortable describing the reasons for the Great War as "dumb". Your mileage may vary.