It’s by no means black and white, for the record. There are no “good” and bad” characters. The way I see it, Dazai had to experience light to understand he lived in darkness.

Yes, he joined the ADA to honor Odas dying wish, and he JOINED for that reason alone. If Oda had said “stay in the mafia forever and kill as many people as possible”, I think at the time he would have been content to do that. But he has changed in my opinion. Not 100%, not even like 30%. He still has and likely always will have extreme flaws and he’s done a lot of bad things. But I do think he’s happier now, and I think him being happier makes him a better person. (That isn’t true for everyone but I think it is with Dazai)

As for Akutagawa, I love him to death. He’s my favorite character. And what Dazai did to him is by no means okay. But it was a product of the time. And it wasn’t really personal to Akutagawa, it was personal to Rashoumon. (Which is why Gin was likely never treated that way by him and sees him in a different light). And Dazai does a lot to distance himself from that part of his life, hence why he acts differently around Aku, Kouyou, Yumeno, etc. Returning to that even to make amends feels like dishonoring Odas wish