Do you think after Dazai first took Akutagawa in he was like how he was in beast? Although much smaller given his age and the panels from chapter 122
(If so I will combust)
Can you imagine this little cutie but even tinier dressed like a victorian child behaving how he did in beast. Do you think he tried to hide from Dazai by hiding under a table? Complained about the paperwork within the mafia? Had to learn to fix his writing? Thought he was the boss? Talked about eating bugs and paper? And just let himself be tossed around like a ragdoll?
He definitely had that rage and lashed out as he did in beast but it's sad nobody was there to quell it instead Dazai tried to correct his violence with more violence when really all he needed was stern but soft disabling and a stable environment