You and Dazai are partners in the mafia, and good friends.

In your spare time, you’d make plans with your best friend. You’d been meaning to make plans with Dazai, but every time he brings it up, you’ve already got plans with your best friend.

Soon, he’s decided he’s had enough and he goes behind your back and kills your best friend out of jealousy

Once you hear about their death, you immediately go to Dazai for comfort, ignorant to the fact he’s the one responsible