for those who don't understand i'm guessing this is what happened:
The evil spirit (the one with black nails) is trying to separate Dohwi from Sungjae since it loves Sungjae. To do this, he tricked Sooyeon into falling in love with Dohwi by appearing in his dreams (ch 18) disguised as Dohwi since it knows that Sooyeon is using a curse that will backfire on him as long as he harbours any emotions besides hatred.
When Sooyeon dies from the curse, Seong-Jae will then turn his anger towards Dohwi since he believes that Dohwi is indirectly involved in Sooyeon's death since Sooyeon fell in love with him. Maybe the evil spirit is using Seong-Jae to kill Dohwi?
idk if this is correct but that's what i've understood from the recent chapters, hope it helps!!
wew it really took him 44 chapters to die