Your suggestion doesn't punish for killing, it punishes for looting. You can kill without looting, and you can loot without killing, so essentially it does not accomplish what it sets out to accomplish. This game strives for realism, so lets think about this realistically. If you have a gun with tons of ammo and you can kill every zombie around you without trouble, and you see someone walking around who could potentially A. have something you want B. kill you in the future, what reason would there be to not shoot them on sight? The only conceivable reason is sympathy, being a carebear essentially. And that's fine, but you must acknowledge that in such a world, those people would probably be the first to die. If you were concentrating, you may see why its so obvious that you should just shoot the person on sight. Its because equipment is abundant, and the zombies aren't a threat. If you had ten bullets, and you knew it would take about 2 or 3 to kill another player, you might think twice about doing it. If you knew that firing your weapon would attract 20-30 zombies which actually posed some sort of threat, you would think twice about doing it. The player element is already there and working well. If you fire your gun and kill a player, and there is another equally skilled player in the area nearby, chances are he's going to kill you. The problem is evident at every level of play, no matter what you are doing. When you go to Starey, or the NW Airfield, or Cherno, are you afraid that you might starve while scavenging? Are you scared that you will run out of ammo during your search? Are you nervous that you might alert zombies in the area? No, the only thing a player is ever scared of is other players. You are going to have a hard time imposing believable penalties on players when chances are, there is nobody else around for miles and miles.