Pretending that the opposite it true, that violence is our most natural and basic state, is equally naive as pretending that cooperation/love is. You mention the phenotype and us/them, but don't explain how violence is an obviously beneficial trait. It is easy to contrive cases where violence is beneficial, but that is a far cry from saying that it is life's most natural and basic state.
Also, I know you didn't actually say this, but I wanted to make the point anyway, since it wouldn't be hard to come to that conclusion from your post.
> Pretending that the opposite it true, that violence is our most natural and basic state, is equally naive
That depends on what you mean. It is certainly true that our natural state is not to be psychopaths, indiscriminately killing everything in sight. But the evidence (and the theoretical foundations) indicate that a certain amount of violence is an inherent part of our basic nature, and so achieving peace requires work.
I do want to emphasize, though, that it can certainly be done. The internet, airplanes, McDonalds -- none of these are part of our basic nature either, and yet we've managed to achieve them. Peace is not out of reach, and indeed, the historical trend is towards an ever more peaceful world [1].
Also, I know you didn't actually say this, but I wanted to make the point anyway, since it wouldn't be hard to come to that conclusion from your post.