The Man
Unbanned zombie poster
From an evolutionary standpoint, homicide is "bad" because it is not conducive to our survival as a species to condone it. Any tribe or group that allowed or encouraged homicide would quickly find itself lacking not just in numbers, but also in certain talents and skills. Killing someone removes that individual's contribution to society, as well as your own. Such a group would die off or be overtaken by its more sensible neighbors. It's not a force of nature that you have no control over, rather your decision to act on the desire to go out and murder someone is something you have complete control over, thus it's a preventable form of death.
Unfortunately survival is not addressed on a species basis but more limited to your local tribe or gene pool. When confronted with other tribes or gene pools vying for the same limited resources homicide is just the evolutionary result of survival, it is really just a matter of whom and how many you kill (usually the most disagreeable). Displays are fine and might intimidate some for a time but the need for perhaps limited resources like water would eventually force a confrontation. In a kill or die situation you are left with limited choices. Their can be additional survival benefits as well, those not killed can add to both the gene pool and skills of your tribe. So perhaps both homicide and a healthy distain for it are both evolutionary traits developed to maximize limited resources and force the integration of isolated gene and skill pools.