
How about you read the story again? Jinha even showed him the naked pictures and confronted him about it. And yeah, at first he didn't know who the psycho was but he knew there was a psycho out there that probably wanted to hurt him. Remember his friend being kidnapped and then Jinha wanting him to stay at his house because he was worried about Siwon safety? But during that time, Siwon thought about "sneaking out". Siwon found out eventually that he was the psycho because Jinha told him after the psycho posted the picture of them making out. What have you been reading, seriously?

References: http://www.mangago.zone/read-manga/a_man_like_you/uu/tr_chapter-57/pg-13/
http://www.mangago.zone/read-manga/a_man_like_you/uu/tr_chapter-57/pg-14/
http://www.mangago.zone/read-manga/a_man_like_you/uu/iur_chapter-58/pg-20/
http://www.mangago.zone/read-manga/a_man_like_you/uu/iur_chapter-58/pg-21/
Jinha didn’t disclose his past with Lu Xuan to Siwon (as of chapter 67). And then Lu Xuan capitalized on the miscommunication and misunderstanding between Jinha and Siwon.
Wow, Go Siwon pisses me off so much. A lot of people say that the story went downhill, but people, may I remind you that Siwon has been hurting Jinha since the very first chapter? Granted, Siwon didn't really know what was happening and Jinha should've said something. I'm not saying Jinha always acted the best every time (because who wouldn't think you're crazy if you come and beat up someone out of the blue without any explanation?) but most of this entire mess is Siwon's fault. Like dude, you know already that there's a psycho out there who wants to break you up and probably wants to hurt you, so why would you want to "try to sneak out"? Or why wouldn't you talk to Jinha when you noticed he was acting kinda off? And why would you follow the psycho and drink with him when you know that last time you went out drinking with him he took pictures of you NAKED! He's such an idiot! And is he for real starting to fall for the psycho a little bit? Any person with a BRAIN would know that he's been interfering with your dreams with his jealousy stone!!! THAT'S WHY YOU COULDN'T SEE JINHA IN THE FIRST PLACE!!! UGGHHH I'm just so pissed off at how much of an idiot he is I can't take it. Idk if I'm gonna come back and finish reading this after the hiatus is over. The only time Siwon was kinda likable was when he was a kid, seriously.