Hyungseok is actor with half blond/half black hair. Early on, he did nasty things to Yoohan on Myungshin's orders. Yoohan has been stroking the guys ego, encouraging him to try to take Myungshin's position as the devil-chairman's pimp/cocksucker. Myungshin found out Hyungshin has been making moves to try to take his place and he had the actor beat up. Hyungseok is not the most upstanding guy, and he has no love for Myungshin, so setting fire to Myungshin's car seems like something he would do
Myungshin will be forced to take out loans now to buy a new car, and he has Yoohan's former loanshark's business card. If he takes a loan with the loanshark, it's unlikely he''ll be willing to work his hands to the bone like Yoohan did to pay it off, plus the crazy interest, so he will be forever in debt. This is a major part of Yoohan's "payback" for what Myungshin did to him
It was absolutely Hyungseok, but Yoohan kind of pushed him into doing it. I was just skimming through and found a scene in ch. 76, after Hyungseok was beat up, and then Yoohan suggested that Myungshin was responsible for killing Hyunseok's chances of being hired for a photoshoot. Yoohan then said to Hyungseok -
"Song Yoohan's [Myungshin] anxious after switching his car. If something ever happened to that car... now that would be worth watching. He'll be crushed to the point of no return."
so who destroyed that bitch's car anyway??