What I would suggest is that if you kill somebody, your body starts burning calories faster, stacking the more people you kill and slowly going back to normal. This way people would actually think if it's worth killing somebody or just robbing them, letting them go, would result in much more interesting survival situations and reduce the worlds slowest deathmatch. It would also be realistic, as humans in extreme sitations (like killing somebody) do waste a lot of energy over a short period of time.