Because Akutagawa is a character that, in a sense, operates with a lot of self autonomy, (ignoring direct orders, working alone, not acting pleasant for other people’s sake) without regard for other people, especially with the context of how Akutagawa is one of the most thoroughly exploited characters in the series (subjugated to Dazai, to Mori, and to Fukuchi in various ways), there’s is something so tasty about Akutagawa being stripped of all self autonomy, respect, and control that he has fought tooth and nail to keep against every other power in his life. He’s never had anything going for him! So to take away the he try little he has for himself would reduce him to a state of not even human. Forced into a state where he is helpless against the forces that harm or subjugate him in any way is the worst (best) form of suffering Akutagawa can go through because it will inevitably break him. He’s not even human to the eyes of many, to deny him further would be to not even see him as an animal.

Tw: mentions of noncon, (male) pregnancy, abuse, mental break

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My favorite way of making Aku suffer is in an AU I made in which Fukuchi not only uses Akutagawa as a pawn as a vampire after killing him, but also as an incubator, forced to birth more children with vampiric mutations. The idea is that a vampire that has both the physical prowess and capabilities as the Hero of the World and an incredibly dangerous and versatile ability as Akutagawa would create incredibly powerful tools for their establishment of a new world after they take it over. Akutagawa can’t do anything as a dead entity with no will or thought of his own, unable to fight back and unable to speak. Two years after, when he’s found and brought back to life—or, well, a state of being able to make conscious decisions and movements of his own at any point and time—Akutagawa mind breaks under the stress and trauma of the abuse and violation his body has had to go through. He has to be stopped from killing his kids (twins) (they look exactly like him minus the purple eyes) very often because they remind him of the man that ruined him. Most of the time he stays in his bed or in his room in the infirmary and stares at the rotting yellow wall paper for hours on end. The events between dying and coming back to life breaks him.

Atsushi takes care of him and his kids, at least. Knows this is something he cannot just “fix” but tries so hard to bring back the fiery sparks in his lover’s eyes. Tries to open up Akutagawa and find him most ruined—love him there. It’s only upwards from there.