What did he do?

I want to hear your theories...It's been hinted at several times throughout the story by several characters (Sohee, Minji, and Mincheol himself) that Mincheol did something to hurt Haneul and that's how she got the scar on her wrist.

Whether directly or indirectly, what do you think he did (or maybe didn't do) and when do you think we'll have a flashback to what happened (since this seems to be a huge part of his guilt about dating Haneul)?