Let's remember that dude managed to evade the police after he kidnapped Donghyun and on top of that shut an entire college up from bullying Siwon. It's obvious his family has a lot of influence. However, Siwon wasn't very smart to ride to the bar with him... but he probably assumed nothing would happen to him in public. Also he wasn't in a very good place to begin with because he was depressed about falling out with Jinha...
HONESTLY though... Jinha should have told him sooner about Xuan then they could have came up with a plan to expose him on media or something. I don't know, no one really thought anything through. Maybe they underestimated him.
"he probably assumed nothing would happen to him in public" Uuhm, did he also assume that he would be taken to a public place?
I don't know, this is just awful writing imo. But people really dig this sh*t so there's that.
Looking back at that chapter and reading what I just said... we are only assuming he rode with him. He could have just as easily met at the bar with him because there is a skip from where he said "wait" to where they are drinking at the bar. Still wasn't a wise decision to meet with someone who was obviously dangerous but I just felt he had super clouded judgement at that point.
If Siwon could have just reported the psycho to the authorities, he wouldn't have been kidnapped and harassed AGAIN. Why does the creator let him continue to be dumb? He could have filed a temporary restraining order from that cunt.