beta
I think he’s just sad because he THINKS she doesn’t love him as much as she used to. So, instead of being normal, he went to fuck some girl to make her want him again (hence, the whole thing about her always crying for him, always wanting him). However, what’s sad is that Haesoo always wanted him. She held on to that garbage because she didn’t want to lose him, and yet it happened.
Side note, authors for whatever novel, manga, etc. should not dictate how you perceive the story. If your mind changes solely on the fact that a man wrote it, you should rethink your values. Hope that helps!