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…
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.
We have more than enough resources to go around for 10 billion people.
The limiting factor is in intelligence and dexterity. In other words, we get richer when we are more.