
so he'll have to interfere with han gyul's love life to be with hg's brother??

Yep, bc he can't get a lover due to bad karma (divine punishment) for having done too many bad deeds. To solve that he needs to make up for it by doing a a good deed, and cutting the thread connecting hangyul and blondie (without killing blondie) is a good deed because it's a fake, cursed thread made with dark arts, which would eventually get blondie and hangyul killed (since the evil spirit that made that connected thread will eventually ask them to pay the price, which is hangyul and blondie's lives). If he can't find a way to cut the thread, then option 2 is to at least save Hangyul alone, by killing blondie.
waited for a week just to see this gray haired idgaf about btch. mc is annoying but i have mc bias so i need to see him asap idc