@justforamin The reason he gave her was that her father ordered
his father murdered. Soo-won's aides add that they supported him because Il was a lousy king whose passivity was endangering the continued existence of Kouka. So a combination of revenge and pragmatism.
Which reminds me that one of the reasons why everyone is so ready to jump on the "King Il is a brainwashed cultist" bandwagon is that a sizeable part of the readership wants Il and the church to take the blame for everything, so Yu-hon, and by extension Soo-won, can be redeemed for taking righteous revenge. (Never mind Yu-hon's war crimes.)
I think those readers' gamble will come up snake eyes. I'm pretty confident Il's decision to kill Yu-hon will turn out to be justified. Politically, he might have been forced to throw Yu-hon under the bus to secure peace with Xing due to his brother's war crimes. Personally, I think it's got to do with Kashi. She's said to have been murdered by "a rebel". Well, if
someone had reason to rebel against Il, it has to have been the people who thought Yu-hon should have been king instead of him. My guess is that Il found that out and worse, that Yu-hon had been in the knowing of, or even masterminded Kashi's murder, and that prompted him to act. I don't think any of this had anything to do with the church, because it had already been disbanded by Yu-hon even during King Joo-nam's reign.
My personal guess is that Kashi was murdered by Hyoo-ri.
Also, for all the people who are calling Il a freak due to what he told Yu-hon in this chapter, while I also think it was excessive, you have to look at the situation from Il's point of view. Here's your brother who's been haranguing, scolding, mocking and looking down on you because you happen to be an adherent of the country's religion (like your father and pretty much everyone else), and then it turns out he had concealed from you that his bride is a descendant of the same Dragon King he doesn't want you praying to. Add to that the fact that you have clear-cut evidence that the priests
are mediums, that they can listen to the voice of the gods, who by the way do exist (and you might hear them yourself?), and there would be good cause for you to lash out at your hypocritical older brother. While it's true that Yu-hon's reasons to take Yon-hi as his bride, or conceal her lineage, had nothing to do with Il's interpretation of these actions, Il's reasoning is
not far-fetched.
It's pretty obvious to me that at a certain point, Il knew everything that was going to happen, Soo-won's usurpation included. Whether he got this from the gods himself (he was in charge of the Crimson Dragon King Sanctuary, after all), or from Kashi, the fact is that he probably considered himself a vehicle for the inevitability of what was coming: an instrument of fate, as it were. He probably knew what was going to happen to Soo-won, too: don't forget, Il told him Soo-won would meet his end at Yona's hands. Knowing that gives me pleasure by knowing all of Soo-won's efforts will end up in vain, the vicious warmonger.
What's unclear to me is how much Soo-won knows. If he only knows about Yu-hon's murder on Il's orders, then I can at least understand why he took revenge. But if I'm right about Kashi, and he knows that, then he's a much worse piece of trash than I hold him for right now.