Like I said before, I do have an idea of my own, it'd just require teamwork. It would be something along the lines of someone knowing that Demons consider deals to be pretty freakin' important, so if a Demon makes a deal, they're going to keep their end of it. (On the other hand, if the other party breaks the deal, they're screwed because that Demon is gonna come and get them no matter what.)

What the idea boils down to is Shax being trapped inside some form of magical prison, or barrier, or something of the sort by a combined effort. While she could break it and free herself if given some time, they could do something that would make the prison kill her before she could free herself, so to get free she has to make a deal.

Might be a little more complicated and such, but really it's the only way I see her as calming down enough to be with the group without breaking character.