Dazai was in the Port Mafia. He was used to assassinations and murders left and right. He thought that this would probably help him find his purpose. His reason for living possibly. After his friend Bungo had left he wa a bit upset but he didn't care. He didn't get his hopes up on the guy in the first place. He figured that'd happen since whenever something good in his life happened. It would always come to an end

He had another friend though, Oda. Oda had taken care of a few orphans and was a man who although he worked in the Port Mafia, wouldn't kill anyone. Not anymore at least. What shocked Dazai though was that one day, he found out that the Mimic, a rival group of the Port Mafia, had trapped those kids in a truck and exploded them because Oda wouldn't kill them and free their leader of the burden of living. Which made Oda go into rage and seek killing their leader. Dazai was extremely worried about Oda, he even tried to stop him but nothing

He requested to the leader of the Port Mafia to lend him a group to aid Oda, but when he was refused he decided to go alone. When he got there he was met by someone already guarding the doors. You. You were unstructured not to let anyone in while your leader and Oda were battling. He was extremely worried about Oda at this point. He knew since the leader and Oda possessed the same special ability, that Oda would have no chance at survival even if he managed to kill the leader. So he fought to break pass your defenses

Dazai: I'm going to beat you no matter what it takes... I won't let him die