>>103026273Because she goes through periods where she tries to be as consistent as she can with her morals to be a better person and that just leads her into taking others' problems into her own hands. Which usually doesn't end up well.
It's a problem she struggles with constantly, though it always manifests in a way she doesn't expect.
The only reason she knows about Bojack and Penny is because she was shown a tape she never asked for, right after campaigning to hold people accountable for their irredeemable actions. In her eyes, it would be dishonest to pretend it didn't disgust her, so she calls Bojack out on it. She fully believes he doesn't care what he did to Penny because he's so hush-hush about it like the big Hollywood stars.
She reveals in one episode that it's less about bringing justice or shame to Bojack than it is justifying to herself that being Bojack's friend isn't hypocritical to her values. Diane has a big problem about being so rigid with her morals to the point where she doens't try to understand others who aren't so cut-and-try "good" or "bad" people.
Bojack is a POS, but he is capable of guilt which shows he is not completely morally bankrupt. And Diane comes to terms with this near the end of the season. She can't forgive him for nearly taking advantage of a naive girl, but she is willing to help him recover.
Keep in mind that she has a so-called change of heart because she herself is capable of morally corrupt actions. She has never cheated on someone as far as we know, but she enabled PB to, which makes her complicit in his shortcomings. She holds herself accountable and adjusts her morals accordingly.
It's awfully convenient that she has a change of heart when she needs to convince herself she isn't a horrible person, but that's kind of her character. She's a prime example of how generally good and kind people can be blind to the fact they can do bad.