A very soft system similar to the old hero-bandit system (but significantly less polarizing) to identify players more likely to be killers:
Upon killing a player, add a Blood Spatter texture to the killer's gear.
The texture overlay could fade after a certain amount of time is spent in game, or upon the bloodied gear becoming Soaked or using an action to wash with a water source (pump, canteen, etc).
This would encourage different playstyles. For example, some people would enjoy being covered in blood. Others, "heroes", could wear white armbands to show that they have no blood on them, and signify that they are there to help first.