>>280345354
Because he was the only one in the world that knew he had a Devil's seed incubating inside him and no one would believe him if he them that Nayun's brother would become a vessel of the Devil.
Put yourself in Hajin's position, how would you be able to convince anyone that the son of a powerful influential family has an undetectable Devil's seed inside him? Tell them all they are actually inside a novel he wrote? Dude just trust me bro? There was nothing Hajin could do to prove it, no one would believe this rando nobody probably trying to get fame.
When the assassination arc came around, Hajin understood that no one would be willing to take on the hitman job because Nayun's family is that powerful, even the Chameleon Troupe was hesitant to do it, the Boss outright refused to do it until she folded like a wet napkin when Hajin called her Captain and even then the Chameleon Troupe had to vote on it and it just barely got the votes needed to greenlight the infiltration, abduction and
assassination only on the condition that Hajin be the one to kill him.