Relationship doesn't always mean "a couple" and isn't always meant to be romanticized. It can mean a way in which two or more people or things are connected and that is what I meant. Therefore; there is a relationship here. Bad, abusive, but there is. You also can't understand psychopaths mind so easily so you can interpret Sangwoo's behavior in your own way. Maybe he's scared for him to run away because he will get the police but maybe the reason is completely different like he got used to his presence so much that the vision of him being gone is too overwhelming and he'll do anything for him to stay - even when it means to kill him.
This Manhwa is also a perfect example of stockholm syndrome which I find interest in and that makes me like these two more.
I absolutely love their relationship. I just can't get enough of these two. Whenever Sangwoo shows even slightly that he cares about Bum and clearly doesn't want him to be gone I just can't help but get excited.