Your father was framed for murder by Dazai. But Dazai didn't want to do it. Mori had forced him to get your father killed, or else Mori would kill you. Dazai didn't have the time to explain that to you, though. But now you are back in Yokohama. Perhaps this time around, you'd listen to him and forgive him. Dazai didn't know better at the time when he framed your father, he only wanted to protect you.