Interesting theory.
We didn't see Joo-hoon's face or get clue of his crime-motivation but he's quite big, may be about Woojin's size makes me worried if the fragile Siwon has to fight with him.
As even Woojin right now ever knocked Siwon out in just one punch.
The problem is, to whom that douche bag is getting jealous. Does he likes Siwon? Siwoon? Or he's just an asshole who loves to bully. Arg
That envious guy harmed Siwon before Siwon(Taemin) being close to Woojin, so we can take Woojin out first & the hypocrite who obviously envied Siwon at that time is his bro.
Now Siwon is close to the most popular guy Woojin, that envious guy will feel worse.
"He's an extremely jealous rat. He'll get pissed off just by seeing you next to me." <-- The one distributed the CD into allover lockers must be Siwon's bro, Ji-wook who envies him.
But the boss in that CD is called Joo-hoon [Ch.16 pg. 9]. So there're 2 culprit-leaders in that terrible crime then..