Dazai was looking for a person to become his subordinate, someone to train to replace him as head of the Port Mafia eventually. Then he found you, a child who was abandoned to his fate. So he adopted you.

At that time they were in a training session. Dazai had called you to teach you "Something new" and when you arrived he just gave you a gun and pointed it at a man who was tied to a chair.

"Kill him." He said, you had never killed anyone so far.