Taehyung is a pureblood vampire, almost like royalty with how rare and elite/strong they are.

Jungkook is a prodigy vampire hunter who as turned into a low level vampire. The turned low levels eventually go insane over time and have to be killed, unless they get blood from a pure vampire. A rare circumstance.  

Taehyung is fixated on Jungkook and offers to feed him. Even loves him. But Jungkook hates him along with all vampires and has someone else he loves.