I didn’t know that

What’s the meanest thing Diane has ever said to Bojack?

It ruined the moral complexity because this incident isn’t only about abuse and bad decisions, it’s about Bojack’s questionable morality as a person. This whole show is based on a likable and relatable character making immoral decisions and therefore raising the question of good guy vs bad guy. This is also where Diane comes in with her perspective on this; that people are essentially neither good nor bad.