Probably just posses him because he’s strong
Brandon is a manipulative power tripper, and he thrives on ruining other people's lives. He also understands the benefits in holding control over Jin specifically. Among Brandon's subordinates, Jin is one of the most useful and capable he's ever had. We can observe throughout the manhwa that Jin is particularly strong and good at adapting.
In some way, Brandon has trained Jin for a considerable amount of time and officially introduced him to the mafia business, turning Jin from a small and negligible drug-dealing thug into a noteworthy figure in their criminal underworld.
I guess Brandon would be unwilling to let go of such a person, especially since he's spent a lot of time on mentoring AND tormenting Jin. Brandon thinks of Jin as his property. 'One of his own', as he says it.
That's for sure as Brandon is like the type who wants everything like everything should be in his control if he loses something he will do anything to get it back and manipulating Jin and giving him drugs just ruined Jin as whole even after Jin knowing it he couldn't just stop him self sometimes I wondered as Jin has bit of feelings for Brandon in Jin's eye he felt like someone was there to care and look at him but after hitting back to reality he understood Brandon just used him still he couldn't stop in getting away from Brandon that guy is such a venomous poison
I would like to see Brandon's pov as I an curious to know why he keep ruining things for Jin does he wants his love or Jin being his posession