It’s not revealed until later (but novel readers know), it’s the reason this started, because bottom raped top in school and did other horrible things but doesn’t remember because he bonked his head.
That being said, what top is doing to bottom now has gone way past the point of revenge- and what he will do in future is just not commensurate with what the bottom did
Top does more despicable things, bottom does try run away once, gets gang raped again and other terrible things, tries to rip himself, starts to remember snippets of what he did to top, and they end up in this miserable codependent relationship. Bottom is completely broken mentally, he’s a mess and a shell of his former self. We’re already seeing it now, but he’s utterly dependent on top (anxious when top is not around etc), and for top, it’s a prison of his own making- he loved bottom, that turned into this revenge, then obsession, (then possibly a sense of responsibility- he so completely destroyed the bottom that bot can’t function without him, so now he has to look after him). It’s not really a happy ending, iirc, the top doesn’t have a grovelling/remorse arc, they don’t suddenly become lovey-dovey, he just stops torturing him

Well that's what happens when you gang grape a 14 year old stop complaining jiwook