Y kept getting flashbacks to him being raped because the only way he could cope was to think he liked it. That way he's crazy but he's not a victim. So when D is treating him kindly, Y says "don't break me" as in don't make me a victim, which could've also been interpreted as don't break my body - which is what D thought he meant. But Y would much rather he break his body than that mental wall he's been building since that trauma. I think Y left to put some distance between him and D, because the only way he knows how to survive is to be crazy, not loved.
what about maybe telling jinha "hey man if you see this creep, RUN FOR YOUR LIFE" or "hey man, if you see him, be careful bc HE HAS A KNIFE" or any of 10000000 other options