Literature
Avatar-Course of Nature
Zuko realized it soon after his talk with his Uncle. More on the ride on Appa towards the Fire Nation. Realization set in that by the end of the day either one of both of them could die. It wasn't that he was afraid of death or dying, just that he couldn't live without her. How did he not notice this before?
When it came to fight his sister, he wanted to make sure Katara stayed out of harms way. Maybe he did need her help, but this saved her from a 'could be'. And if he could save Katara, he would do whatever it took. Besides, it didn't seem like Azula was herself. Things shouldn't be too difficult.
However, when the lightning came towards