It's definitely not obvious, given that many, many gregarious species may certainly have inter-group clashes and skirmishes at territory boundaries but no full-scale war. Animals in general avoid violence between conspecifics, for the obvious reason that it's rarely worth the risk of being hurt unless you're very sure you're going to win. Dying for your group is something you almost never see outside eusocial species. Never mind dying in your prime reproductive age!

Yes, but war is worse for all parties generally.

Lions murdering prey to eat is a stable equilibrium.

Primates fighting each other is not.

Murdering for acquisition of a resource is short term advantage.

We are strongly, strongly evolutionary oriented away from 'murder' - it's the original sin. It's not something we even argue over. Murder = Bad. No disagreement across cultures. Murder = social cheating. No disagreement there either.

Or put another way - the 'self' can gain advantage with murder, but the group and species probably will pay for it long term.

I wonder if there are just things that species really have to learn over and over, particularly things like 'active deconfliction' etc..

> When competing for resources, killing your neighbour frees up resources, which you can take. Most species of animal and even plants do this to some extent.

If anything, I'd say plants do it more. Everything in the garden is trying to kill everything else.

> When competing for resources, killing your neighbour frees up resources, which you can take

I don't think it's that straightforward. War is usually extremely wasteful for all involved, even the victor. Plus it puts the whole group at risk, if it spirals out of control.