It's just how I see it, but I think that if Zuko had been neglected by his father all of his life, it probably wouldn't have been so hard once Ozai turned away. I think it started when Azula was showing her prodigy-ness, increased even more when Lu Ten died and he saw a chance for the throne, but it ballooned when Ursa allowed herself be banished over letting Ozai kill Zuko. He was going to kill Zuko himself, true, which is also a surefire sign that he had started seeing Zuko as expendable by then, but I think he hated Zuko when Ursa left. That's probably just me being a weird romantic, though.

I never understood why people thought that Mai was being a bitch when it was clearly Zuko who was being a dick. Granted, he wasn't himself, and we all know why, but how was Mai a bitch? All she wanted was for him to fucking chill out.

Exactly. Azula by then was probably so used to being thought of as perfect that anyone who questioned her and anything she did was thought of as hating her. It probably hurt even worse to hear it from Ursa, since it was clear already that Zuko doted on Ursa and she didn't mind.

This family T.T