This is a post about tonight’s episode – don’t read it if you haven’t watched it yet.
So, I can only assume that the “changing the rules” Ben was referring to was the killing of an innocent person. I think that both Ben and Charles are somehow obligated to follow the same set of rules (perhaps “by the island”, whatever that means), and at least one of them is based around the “good people” thing they’ve alluded to throughout the show, specifically forbidding them (personally) from harming them.
Just a quick reaction theory, but I thought I’d throw it out there…