Because the person he loves LIVES with that monster (Jiwook) and can't avoid him, plus Jiwook was the one manipulating Joohoon to do evil, too. In other words, Joohoon doesn't give a shit about Siwon, but Jiwook will never stop making Siwon's life miserable. The one that needs to be neutralized is the one in proximity to the victim and the one keeping the abuse ball rolling. Priorities.
Siwoon can always find a way to get Joohoon...later, maybe But remember, Siwoon is no angel, either. He saw the tape of Siwon getting raped and didn't give a crap. He only cared when it was TAEMIN as Siwon being hurt. He has no interest in avenging Siwon (real one). He wants to protect and avenge Taemin/Siwon.
I was hoping for more chapters where we see Taemin happy and Joohoon pay a price and Jiwook go to fricken hell and scream forever.
I can't be completely happy about this... Because then it means that he worked alongside the monster who got Han Siwon gang-raped. What kind of justice will be that? Only one of the two big ones behind it will pay, and it will be the pathetic one... The other one not only gets away with rape and assault, he even gets a good laugh with it.