Okay, I understand what you're saying now. If Zuko does betray Aang and his friends again, then I could definately see Katara being really upset about it. Based on her personality and the dangerous power she has as a blood bender, I still think that she wouldn't use blood bending on Zuko unless her friends and/or innocent people were in danger and she had no other choice. After the Puppetmaster episode, Katara realizes that she has this incredible power that allows her to become so powerful that she can practically kill people without even touching them. Like I said before, from what I've seen, Katara is a morally-driven kind of person and after seeing what happened to the old woman, I'm sorry that I can't remember her name at the moment, after she developed blood bending, I doubt she would want to risk ending up being like her. While she does function with a good sense of morals, power can change and corrupt even the kind hearted people.
You do bring up a good point about when Zuko didn't take Katara's sympathy and compassion. I honestly think that the tempatation of getting what he's always wanted back, his honor and respect from his father, was just too much for him to refuse. Even if it did mean losing ties with his uncle, for awhile anyway, and the chance to finally heal his scar. I didn't really think that he almost killed Aang in the Crossroads of Destiny. I know that they fought, obviously, but Azula was the only one to really come close to killing Aang during that event. Still, I understand that Katara will not be as trusting as she was to him during that episode. I just can't see her using blood bending on Zuko, let alone anyone really, unless she has no other choice to make.
While I have no connection to the writers or the creators of the show, obviously again, I also can't see Zuko betraying Aang and his friends. Sure, they won't accept him right away, but I'm sure that he will help them out. After all, he did tell his father off during the Day of the Black Sun, told him about how he felt, said that he was going to help the Avatar to take him down and that defeating him was not his destiny, but the Avatar's destiny. Most of the first half of book three also dealt with how Zuko felt after getting what he always wanted. He realized that it wasn't what he wanted anymore since he didn't feel like himself. As for Azula, I can see her trying to toy with him and make him come back, if it works out in her favor, but I doubt that he would really fall for it. Mai will most likely prove to be an interesting factor, though. I actually hope that the both of them get together too since I felt that their relationship was true and that they had honest feellings for each other. Based on how her feelings and how she doesn't seem to care much for being in a in-charge kind of position, I can see her choosing Zuko over the Fire Nation. That or she will just wait for him to come or something like that.
Of course, the writers could throw something completely different and surprise everyone out of their minds. They've been known to do that and it will be wonderful to see how the rest of book three turns out.