The opening post clearly describes that being the wolves should be a PK-role, not only one side of a balanced pvp situation. If the sheep gets a specific role withing the confrontation for example by giving the sheep abilities to escape or making the sheep agree to a hide and seek game, its not the same as if the sheep wants to do something else like mining and is surprisingly ambushed.
A PK requires victims, real victims. It also requires the amount of sheeps to be >>> the amount of wolves. Why should a majority of people want to play the sheep for a low number of wolves? I don't think a concept like that would ever work.
Maybe it gets more interesting when the wolve is not the topmost of the games food chain. Let's add humans.
- Players can kill each other
- Killing players give the PK an 'outcast' status ( he becomes the wolve )
- Wolves can't enter cities and use other parts of the game anymore, that keeps the amount of wolves small
- Hunting wolves gives a nice reward for humans
- Whenever the amount of wolves increase, humans have increased motivation to hunt them down and make the land save again, so sheep can go sheeping again