F1 already knew MC did it with F2 because F2 was bragging about it to F1
uhh confusing but- there was a panel where he says "I have a loud mouth so you have to shut me up" anddd basically that right there is when F1 knew everything.
Too answer your question and yap a bit
It's (MC) Haewons fault
said by F1 you can see the answer for yourself in the bathroom scene and *cough* others... is that his WHOLE reason for doing any of this is because "its Haewons fault" all because he did it with F2 first before him. thus all the torture and violence.
F1 is uhh insane, crazy, and should probably die from my understanding from the beginning he thought MC and him were dating to begin with (novel spoilers) since F1 knew about MC little secret kisses and knew he liked him just want to play with his feelings for a bit I guess, basically what I got he just wanted full on control of MC since he seemed REALLY HAPPY when HE CONFINED MC for oh how many months? LONG ENOUGH FOR THE SEASONS TO CHANGE!!1!!
(novel spoilers) plus has to why MC doesn't leave is because F1 (SEO HAEYOUNG) has basically brainwashed him from his early years to ONLY RELY ON SEO HAEYOUNG hence the "3 rules" and that's why our pitiful MC lives happily ever after with F1
..Wtf.....?
So from my understanding Fried1, who MC has a crush on and is super rich, had been in delululand thinking all this time him and MC are already a couple and are dating. So when Friend1 found out MC has sex with Friend2 (black haired guy), he thought MC had cheated on him. And because in high school he misunderstood that MC only listen to him after he is beaten, he decided to make MC go through all this hellish stuff to make him realized that cheating is bad. So Friend1 beats and torments MC because he thought MC was simply a slut that has sex with everyone because he's addicted to sex.. Thus he kept beating him and get others to r*pe him because this way MC would eventually hate sex with other people. And got Friend2&3(dark skimmed guy friend) to assault MC because he thought MC liked them aka Friend1 was jealous and didn't want MC to enjoy sex with the other 2 so he purposely made their sex times with their 2 friend more traumatic and non-con for mc? I'm confused.... How could Friend1 handle MC having sex with others if he's this jealous and toxic?