I haven't read the Beast novel so I can't judge this manga as an adaptation. However, this story isn't your average reverse AU fanfiction; the premise serves as a basis for exploring the psychology of Akutagawa and Atsushi. Dazai is mostly in the background but he is still essential to the plot. Aside from the more character-driven narrative, this manga also differens from the original one in art style and tone. While I prefer Harukawa's art style, Hoshikawa's style is just as detailed and dynamic. Furthermore, this artist does an amazing job of conveying the characters' emotions. They also use stronger contrasts, which fits the darkertone of this story. Speaking of which, this manga is much more brutal than the original. (yet this is shounen?) It feels so excessive that the shock value becomes lost, which kinda undermines the story's message.
Thankfully, the narrative and themes leave a bigger impact than the violence. This manga makes you see Akutagawa and Atsushi differently by exploring the parallels between them, not by switching their personalities. They both deal with trauma in unhealthy ways and do terrible things, but we root for them to change, even though they think they can't. This is a story about revenge and regret, about self-acceptance and overcoming one's past. It shows us that it's never too late for us to change and that we can still find happiness.

I think BSD is at its best when it tells street-level stories like this. It's so much more personal than the chaos that the main manga has become. Even though this story doesn't revolve around a case, it's still one of my favorite stories from Asagiri. I'm going to miss this alternate universe.