JW(Jiwook) gang-SAs JY(Juyeon) for a while in high school and spreads JY's SA photos and videos everywhere and sent them specifically to JY's mom, causing her to mentally break down and end up in the psych ward. After he gets bored with JY, he started SA-ing other younger boys for fun including Lee Sangjin's (one of the dudes who gang SAs JW) little brother if I can recall from the novel, can't remember exactly if that's correct. This all started because JY confessed his feelings to JW and unfortunately JW was a huge homophobe.
Near the end of the main story after JW finally remembered what he did to JY, author implied JW was a closet gay since he revealed he actually liked JY back after his confession but was so afraid of admitting them that JW decided to lash out in the worst way possible and accepts the abuse he gets from JY as repentance.