
Everyone knows that at some point he had innocent feelings for Sangtae, the manhwa explicitly shows this. Everyone also knows that Sangtae right now still "loves" Jinwoo, though it's been twisted after years of obsession. It'll get better once his "love" is reciprocated, it'll revert back from obsession to love if he can feel secure in their relationship. What everyone has been saying is that his feelings don't justify his actions. He has long since moved on from the level of "notice me senpai" antics to full-on harassment. He has made Jinwoo's life a living hell even at the beginning of the manhwa, there's been years since highschool with Sangtae harassing him and following him. Jinwoo didn't hate Sangtae in highschool, he was a very gentle guy, but he hates Sangtae by the present time. Now he's sexually assaulted and raped (he fingered him against his will, penetration) him because he got horny after seing his crush disabled by the fantasies, which traumatized Jinwoo to the point that he couldn't have sex with his gf and now he is "a shell of his former self" as Sangtae says. To give some context, what Sangtae did is the same as seeing someone drugged with a date rape drug and saying "i wanted to help them, but they were hot so i couldn't help myself".
I've said all along Sangtae as loved jinwoo. Everything Sangtae as done, was to get Jinwoo's attention.
“from the moment I first set eyes on you I haven't been able to stop thinking about you. Every time I look at you I'm reminded of how I can never stop loving you.”