The primatologist Richard Wrangam once advanced the theory that tribe vs. tribe conspecific homicides - what he called coalitionary killing - are an evolved trait that was selected for in primates by some kind of pro-homicide selection pressures in the ancestral environment (where homicide reliably grants an advantage to the expected relative gene frequency of the perpetrator's genes).

I haven't kept up with biology for years and don't know what the current consensus on the topic is but it's interesting to consider if some environments naturally promote the unlucky inhabitants to harm each other.

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.