Since the very beginning it was obvious the author hasn't been aiming for a simple story with easily defined heroes and villains. I honestly don't understand why some people are upset that the story makes Yuhon relatable.
I would also be extremely upset if it were revealed that Hak is Yuhon's son. Firstly, it's poor storytelling, when everyone is blood-related to everyone. Secondly, relationships between cousins are only slightly less gross than between siblings. In my opinion, being cousins could be enough of a reason for King Il to try stopping Yona for developing feelings for Soowon, but perhaps he was aware of the Crimson Illness, as well.
It may be that Soowon's mother sickness is crucial for the whole story. Like, she becomes good friends with Il and tells him about the sickness, which makes Yuhon jealous and ruins the relationship between brothers. Or that when Yuhon learns about her sickness he becomes ruthless, looking for possible cure. Or the illness makes Yonhi sick physically and Yuhon mentally (and the same would happen to Yona if she had a relationship with Soowon). Perhaps Il later kills Yuhon because Yonhi begs him to do it. Something along these lines... The author has a magnificent talent of writing a story which is at the same time logic, moving and surprising, so I suppose my guesses will be wrong, though