
The future chapters will reveal more of his past. He got messed up by childhood trauma because I'm pretty sure (I could be wrong) he was attached to his mother and there was a scene where he's following his mom and she fell from the stairs in front of him (either he caught her by surprise or she slipped) and he reached out his hand to grab her (Geon mistakes Yul of pushing their mom when he was really reaching for her) after that happened he started having nightmares but Kyungsoo would rub his head telling Yul that when he would rub his head, Yul would look comfortable like if the nightmare went away. After he let his friends do that instead of having nightmares about his mom they got replaced with Kyungsoo.

I think Yul has a mental illness of some kind, He is obiously sociopathic by his lack of empathy & feelings, but He knows how to fake it. And He is narcissitic too ("kyungsoo fell for me because of my looks").
in addition, He didn't seem to feel anything while the gangrape was happening, ony afterawards, I guess He remembered his mother death when Kyungsoo shouted for him? and now He related both incidents...
Or maybe He actually killed her without realizing it, so He shut off completly after that, and after the Gangrape he started to feel again.. only it is all misery, because of what He did.
Anyhow, I think He is really mentally sick and needs profesional help.
like, did he really push his mother and kill him? Was he fucked up since birth or was he fucked up due to a childhood trauma?
I also wanna know if he's had his friends rape his gf/bfs before Kyungsoo (I dread to imagine how many people they've done this shit to tbh.. but it's entirely possible)
I feel like he's been mentally sick since a long time ago and it just keeps getting worse because of his spoiled lifestyle. But because his mother is dead, no one really cared enough to know that he's sick. His brother hates him and his sister just tolerates whatever Yule does because she's also sick or she doesn't really care.