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preeettty sure the "reason to live" dazai promised akutagawa was atsushi.
akutagawa stopped killing because atsushi told him to. akutagawa gave his life so atsushi would keep living. atsushi couldn't wholeheartedly attack akutagawa because he was scared of really hurting him and never doubted he was still in there because he saved a girl.
atsushi knew akutagawa was still in there because akutagawa, rabid dog of the port mafia, didn't kill.