I’d have to say, during their stay at the Western Air Temple, either before or after the Boiling Rock episode. So you can add self-loathing and guilt on Katara’s part.
But, the thing is, it doesn’t change the way she treats Zuko. If anything, it gets worse. All of a sudden, the Fire Nation is much more despicable and ruthless than ever before. If they did that to their own Prince, what’s left for the rest of the world? This realization brings a lot of issues into the light. And she takes it out on Zuko, because she can’t process all of these issues all at once and turning his face into the enemy is familiar, it’s safe.
Zuko, on the other hand, doesn’t know how to react when Katara reveals to him, in the bring-my-mother-back scene, that she knows.
There are a lot of conflicts to resolve during the Southern Raiders trip.
I can also imagine she wouldn’t know how to process that Zuko did all the horrible things he did for a man who did that to him. Everyone tends to assume that the gaang, once they knew, would immediately understand, but it’s not something easy to understand if you’ve never been through it, how someone can continue to be loyal to someone who hurts them like that. So she’s angry at Zuko because she’s sad for him, and that makes her angrier.
This is Katara we’re talking about, who got angry at Sokka for not being as angry as she was at their mother’s murderer. Katara is so angry at Ozai for what he did to Zuko that it actually makes her angry at Zuko that he isn’t angrier at Ozai. She’s angry at him because she offered him healing and he still betrayed her to go back to that. She’s angry because she doesn’t want to acknowledge how much it scares her. Because if there is one thing that scares Katara, it’s the idea of being a victim. And Zuko isn’t like the people she’s used to taking care of. He’s like her. And if someone like her can be a victim like that, then it means she has to acknowledge her own victimhood, too.
