Seogeum wasn't trying to manipulate tae in he did cut himself but it's only to make tae in remember him as a psycho and leave a stain in his life. Tae in loved this crazy devotion from Seogeum because it's not manipulation
Doing something drastic to intentionally leave a mark on someone is manipulation. You are manipulating the way they see and feel about you. Just because tae in ended up falling for him doesn't make it any less manipulative, it just makes tae in equally insane in a different way. It is absolutely psychotic behaviour to mutilate yourself in someone else's house for any reason.
Ok whatever floats your boat
It's true that seogeum's behavior is psychotic but he could've easily manipulated tae in by crying his eyes out and begging tae in to take him back or give tae in his full consent to wreck his body as much as tae in likes but he didn't because he believes tae in really broke up with him so he chose to mutilate himself since it's the only way tae in will not forget him that's how twisted seogeum is for tae in.
Giving it a long-winded explanation doesn't change the fact that it is manipulation. Just because it doesn't look the same as what you think of when you hear that word doesn't mean it isn't.
Literally can't stand either of them I especially can't stand people who use harming themselves as a manipulation tactic. I knew someone like that and I had to accept that if they went through with it just because I left then that was not my fault or my problem. I guess these two were just a match made in hell