Honestly I've been toying with the idea of a betrayal but since he's been in the party he doesn't want to kill them because they helped him achieve his goal of usurping or taking the power of whatever they're after. If they try to stand in his way he mames or injures them severely if he succeeds obviously and that also gives motive to the other player characters to hunt him down. I mean if someone cut off your arm or took one of your eyes when you want to get revenge? I think you could easily have a player character become the big evil person and that person could even actually play that role and actively respond to what the other heroes do.