Even if King Il actually kill Yu Hon because of the latter cruel way and So Won know it, it won't change anything. So Won still will betray King Il and Yona because he thought that his father deserve the throne more than King Il. I believe that he also think that he's doing it for Kouka kingdom. So, So Won betrayal and King Il assassination are bound to happen sooner or later. I also believe that So Won betrayal is necessary for Yona and probably part of her test to become the real successor of Hiryuu. Yona will still be in the palace and won't meet the dragon without this event. She also will never look toward Hak way because she will be too blinded by her passion for So Won. That event is important for Yona development. If else, why Zeno just be silent? I believe that Zeno always watch upon Yona as Hiryuu reincarnation. He watch for a long time until he's sure that Yona is strong enough and ready. But, as much as he want to test Yona and see her worth, he won't take the risk anyone killling Hiryuu reincarnation. I think even Zeno know that the event is necessary and it's also the only way for Yona to grow up and being freed from the palace.
Even if King Il actually kill Yu Hon because of the latter cruel way and So Won know it, it won't change anything. So Won still will betray King Il and Yona because he thought that his father deserve the throne more than King Il. I believe that he also think that he's doing it for Kouka kingdom. So, So Won betrayal and King Il assassination are bound to happen sooner or later.
I also believe that So Won betrayal is necessary for Yona and probably part of her test to become the real successor of Hiryuu. Yona will still be in the palace and won't meet the dragon without this event. She also will never look toward Hak way because she will be too blinded by her passion for So Won. That event is important for Yona development. If else, why Zeno just be silent? I believe that Zeno always watch upon Yona as Hiryuu reincarnation. He watch for a long time until he's sure that Yona is strong enough and ready. But, as much as he want to test Yona and see her worth, he won't take the risk anyone killling Hiryuu reincarnation. I think even Zeno know that the event is necessary and it's also the only way for Yona to grow up and being freed from the palace.