I didn't read the novel but from what I've seen in other comments the MC also gang raped the ml when he was still a teenager so what he's putting the MC through now is revenge and probably partially out of frustration that the MC doesn't remember what he did. Also while it is hard to see what happens to him, I don't think a rapist deserves any compassion ever
I asked the same thing a few chapters back, and someone was kind enough to reply with this:
Jiwook gang raped Juyeon in high school. They took pictures and sent them to Juyeon’s mom, who suffered a psychological break that she’s still in the hospital because of. Jiwook got into a fight with one of his “friends” and was beaten so badly he has complete memory loss of what he did to Juyeon.
Tbh they really are made for each other. I don’t even know whether I should feel bad for the top or the bottom anymore. They’re both shit

I read some spoilers as to what exactly happened in the past that he treats him this way and ngl I'm stuck idk what to feel after knowing the past. He kinda deserve it?? Idk anymore