"Now that I don't agree with. I think she left because of whatever this deal is she was trying to work out with Weaver in the future."

I know that, I said "in part". I think it helped her decision to leave John's side.

I'm going to express the theory that's directly opposite of yours. :) Cameron gave John the watch to kill her, then she explained to him that she still might go bad. But after she makes him lie on top of her and feel her symbolically ice cold heart and sees that he's still very much infatuated with her, she realizes he will not be able to kill her if it comes to that. So she decides to leave him behind as she goes to join Weaver's cause.

There is one issue in any case. Cameron would have to know that Weaver has a time machine.