Earlier quoted context omitted.
It seems obvious to me - it's the combination of two ideas: 1. 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. 2. By collaborating in a group, you can achieve more than individuals acting alone. This is the idea behind teams, companies, countries, etc. Combine the two ideas, and you get war.
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 disagreem…
There are plenty of people who advocate for war and consider it good, and plenty of disagreements over war.
People are usually in agreement that war / killing is bad when other people do it but will find all sorts of ways to justify themselves doing it when it is to their advantage. This isn't really contradictory, from an evolutionary perspective.