When Azula attacked the Western Air Temple and Zuko fell while fighting her, Katara could have reached out and caught him…
But he was too far away, falling too fast. That's what she told everyone.
They were better off without his treachery, anyway. Even if Aang hadn't finished mastering firebending yet.
Because Aang wasn't ready to face the Fire Lord before the Comet came, they decided to wait in hiding… But while they waited, the Earth Kingdom burned.
Now Aang is dead, so many are dead, and Katara is Ozai's personal slave…
She wishes she'd reached out to Zuko again.