A humanity meter would be pretty nice. It would solve the problem rather elegantly if you simply gave respawning benefits to humane players, and harsh penalties to murderers. For example, an extremely humane player could spawn with a mosin and some ammo, and a serial killer would spawn without a flashlight and starving. The tradeoff would then be between starting off with better gear and being able to gear up fast by killing other players. The problem however is deciding how you gain / lose humanity. You can't just say "well if I kill a player then I should lose a lot of humanity", because what if you killed him to defend yourself or one of your buddies? What if I just knocked someone unconscious but then looted all his gear, potentially sealing his fate? Any system you put in place to detect who started a fight or to gauge the severity of players' actions would be necessarily complex, flawed and open to abuse, because players are crafty. I don't think there is an elegant solution to this problem.