Leave it as is. The human element of kill or be killed, risk vs. reward, and altruism vs. desperation is just as integral to the atmosphere of the game as the zombies. Having other players that can either only be friends with you or ignore you is basically just static co-op. If you were killed, then you were either ill equipped or just in the wrong place at the wrong time. If you really must have no pvp (which really only means that instead of fighting people for life-preserving resources you race them for it), join a private server with a dedicated admin that instabans PKers.