it is the whole story spoiler!!!!
JW(Jiwook) gang SA'ed JY(Juyeon) for awhile when JY was 14 and JW was 17-18. When JW got bored, he spreads the SA photos and videos everywhere, and JY’s mom sees it causing her to mentally breakdown and end up in the psyche ward. This all started after JY drunkenly confessed his feelings to JW, but unfortunately JW was "homophobic" asf.
JW then lost his memories after a fight with Lee Woojin (Dude JW beats up in ch 19) and JY visits JW in the hospital, still willing to forgive and hoping for a fresh start. But I think Lee Woojin or his friends tells JW about the rumors that JW had SA’d a boy before he lost his memory. So when memory-loss JW asks JY who he SA'ed and if the rumors were true, it triggers JY’s PTSD and from JW's reaction, JY thinks JW figured out it was him and has a full blown panic attack and runs away. Lee Woojin bumps into JY in the hallway of the hospital and tells JY he should get payback. JY thinks about it, and decides to go back and ask memory-loss JW rhetorically what he would do if someone was bullying him. JW says he would return the favor tenfold. So that's what JY decided to do.
Near the end of the main story, JW regains his memories and author implied JW was a closeted gay who actually liked JY back but was so in-denial about it that he chose to lash out in the worst way possible. He then begs JY not to leave him and accepts all JY's punishment as repentance
