I think what broke Azula was Mai and Ty Lee betraying her in The Boiling Rock. THAT was the turning point for her character. She tried to brush it off, say they weren't really her friends anyways, but I think that's what slowly began eating away at her and why she feared betrayal from everyone in the finale. I think even Zuko and Katara said that Azula's attack on the Air Temple, before the finale, was very out of character and reckless for her. I like the way it was portrayed, added a real darkness but also sadness to the story and her character.