Tbh the only thing stopping Akutagawa from just killing him is the idea that Dazai would be upset. Otherwise, the deal just seems really appealing, knowing that Aku already wants Atsushi dead anyway, though in the first place Aku's hate towards Atsushi is rooted in the fact that Aku believed that Dazai choose Atsushi as a new pupil because Atsushi is "better".
It's possible that Aku's feelings have changed since Dazai came clean about his plans for "new double black" in the recent chapters. In that regard, Dazai really is a genius if he anticipated this proposal that Aku kills Atsushi, and decided to tell Akutagawa about it just to prevent that very incident from happening.
Though, personally I think it would be honestly be kind of refreshing if Akutagawa "betrayed" Dazai for the first time-- IMO Dazai would kinda deserve it. I don't necessarily hate Dazai, but it's evident that he takes major advantage of Aku's respect/admiration towards him, so it would be interesting if Aku decided to grow out of that for once.