Violence is an effective strategy. One of many. It's not stable but it has been used successfully in human history. The thing is you can't have a society that's constantly violent all the time forever and ever. Those tend to self select via natural selection.

You can have societies be temporarily violent like how Americans slaughtered and killed Native Americans and took over the continent. The formation of the USA comes from this type of effective violence. Who would be the alphas in this case? the native americans? Or the ones that invaded? I know I'm a bit on the nose here, but this is just reality.