after riko’s death cause of toji and you couldn’t protect her in time you started becoming depressed and didn’t know who you were stopping curses for. You started killing non sorcerer because they are the cause of cursed spirits spawning even though it’s not their fault. After you killed your family and 112 people you became a cursed user. As you left jujutsu tech everyone heard of what you did shoko was in the city looking for a lighter for her cigarette when you came up and offered her one