You call it writers screwing up.?? I call it irony created by foreshadowing.?? I remember reading somewhere that there has been too much evidence to Zuko changing that it would be surprising if he remained loyal to his father.?? And they were indeed correct.
What caused him to remain loyal?????It was more the failure of the protagonists to lure him over than it was of the antagonists.?? Zuko has come to the realization that there is a better chance of restoring his Fire Nation honor than being accepted into the Earth Kingdom/Water Tribe.?? In Book 2, Chapter 7 ("Zuko Alone"), after he defeats the Earth Kingdom soldiers, the town forces him out after discovering he is from the Fire Nation.?? Heck, Katara, arguably the kindest person in the series, is yelling at him earlier in the episode.?? She then explained it best, Zuko is "the face of the enemy", even if he doesn't want affiliation with the Fire Nation.?? He realized that people in the Fire Nation understand him the most, and that, I believe, is why he remained loyal to the Fire Nation.