On lots of mmorpg's, pk is free at all zones ( like MU online ), only thing you loose is reputation or u get status changed to killer for a certain amount of time.
Thing is, how many times u saw someone koxping and couldnt do shit beside trying to pointlessly lure a mob on him. Or when u are doing a boss and someone trade spam u or lure on you priest, u get angry but u know that there is nothing that u can do.
My sugestion is to make killing people avaliable at all zones (except inside towns and cz), not for nps but with a small penality for killing someone.
just an idea .........