I assume he want's to get back at Joo Seungeon (the black haired guy) for some grievance in the past and he wants to use Jiwook for that. So he purposely get's close to Jiwook as an object, just to harm Joo Seungeon by eventually threatening Jiwook. Though he's also turned on a bit by him because ... plot.
He's also very sadistic (we don't know why, it's just .... plot.) which tells us that the uke is in serious danger, so Joo Seungeon can have his great, dramatic moment, I guess.
Both treat Jiwook badly. But the Blondie is depicted as the really REALLY bad guy, so Joo Seungeon doesn't look as bad next to him. I don't know if it's the author's kink or purposely written like that.
Though I am still hung up on: WHY does Joo Seungeon like Jiwook to begin with? :| How did he capture his interest and heart?
Jiwook: *exists*; Joo Seungeon: *heavy breathing*