I disagree that it is innate natural behavior. Killing isn’t socially acceptable behavior in fact killing is the most antisocial thing a person can do. And diplomacy is the opposite of violence.
"Socially acceptable." That applies to within ones perceived tribe. That also only applies to killing for ones personal reasons. Killing outside of the tribe, directed by the tribe isn't called murder. It's call patriotism.
A different tribe, a different religion, a different color, a different language and it's game on.
We have records of war as far back as we have writing. Battle of Kadesh, Troy, etc. Over population wasn't a issue then. I remember reading about neolithic sites in Britain where evidence of a last stand of some tribal group in a village bounded by earth berm fortification. Littered with arrow points an skeletal remains. Men, women, children. I'd be surprised if there were million people in all of Britain at that point, spread between Cornwall and Scapa Flow. Somebody thought someone else was the "other".
I have my suspicions on what happened to the Neanderthals.
The rat experiment you mention is an example of what I would call of intra-tribe warfare. Even the tribe can breakdown under extreme pressure. A similar case on Easter Island in it's past.
So one big human tribe? Nope. If a tribe gets big, there is an inherent human reflex to begin to fracture into sub-tribes. North vs South, Balkanization, etc.
More human that opposable thumbs. Always has been; always will be.
