Jiwook made Juyeon and his mom go insane a long time ago. Lee Woojin(?) (ch 18) manipulated Juyeon into taking his revenge to use Juyeon for money to get out of his gang and pay his debts when initially Juyeon was willing to let bygones be bygones.

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. 

If there hadn't been photos of him spread around, I feel like he wouldn't have gone this far. Juhyeon was also gang-SA'ed every day, and the videos his mom saw made her go insane, so I feel like that's worse than what he's putting Jiwook through. He wasn't even gonna take his revenge at all until one of the dudes he called to mess up Jiwook encouraged it.

Jiwook gang-raped the Juhyeon when they were in high school and apparently had been doing the same to other kids younger than him anytime they pis*** him off. He then lost his memories in a fight. Juhyeon was willing to forgive him even after all he'd been through and even visited Jiwook in the hospital after he lost his memories. I can't remember what made him snap. Probably the fact that Jiwook showed that he was still the same bas**** even after losing his memories or one of Jiwook's rival convincing Juhyeon to take his revenge.

Initially Juyeon wasn't planning on getting revenge but Woojin insisted that he shouldn't let it go. If he took revenge on his own, I don't think his actions would've gone this far.