In PvE, there's still a way for players to kill others... by turning into zeds after being infected.
What I suggest is this... what about a button similar to the "PvP" button, that would allow YOU to be harmed, while you can't harm other players?
Like say, for example, you get bitten by a zombie, but manage to return to base. The fever has already hit you, and you tell your friends, "You have to shoot me. I don't want to turn on you guys..."
Having this as an option would allow some pretty interesting scenarios for people in the PvE side of things, without the risk of turning a server PvP JUST to allow "Mercy Kills". Heck, y'all could even use a "Target" icon since the skull's already been done. ^_^'