[quantum_mechani]

Arryn said:

Well-reasoned, but slightly flawed. What actually happens is that the aggression is turned outwards towards other tribes. Evolution selects for the violent ones, as it is their genes that tend to survive while the "weak" get killed off by those more violent and capable, be it in a bid for power as to who'll rule the tribe, or when tribes war against one another.

It's true that cooperation is the better strategy, but it almost always takes a back seat to the exercise of sheer unbridled power. (The U.S. invasion of Iraq is the latest example of this, and on the largest possible scale. However noble the intentions may have been, and there's a lot of room to doubt even that, it was still morally wrong.) Historically, people have cooperated only so long as they are getting what they want. If they cannot, or cannot get it fast enough, they'll resort to violence in spite of what that violence may invoke in the way of repercussions. Criminals, dictators, and even some ostensibly-elected officials all do this.

You can always take it to a higher level, and say that a species that on the whole does not kill each other does better. And even if your theory of strong tribes killing weak tribes is true, our aversion to murder would seem a good way of deterring tribe-internal killing.