"Can you expand on how the IPD shows the need for government? I don't see it."

I shouldn't have used the word government. That is more of a social development. What I meant was just some kind of code or agreement between people to keep order. It would stem from the need to survive as a people - to keep everyone from killing each other. I imagine that as man's brain developed beyond animals', he began to discover the advantages of reciprocity, and the "optimum equilibrium" the website mentions. Government, morality, and all that other stuff came way later.

Keep in mind that I haven't answered your original question. I've only offered a theory as to how revenge affected the evolution of man, not a justification for revenge.