Yona was told by Soowon that not all deaths are as they seem, which makes her wonder about the death of her mother, who was supposedly killed by bandits. The mother was revealed to be Kashi, a surviving priest (before Iksu was revealed). We know that Yuhon attempted to have all the priests burned but since Kashi was saved, he may have eventually found out that she survived the massacre. This leads him to kill Kashi, infuriating King Il (for reasons that may perhaps not be love, but rather use, as Kashi can see others' futures). As "revenge" or a spur of the moment, King Il kills his brother, in which Soowon is later told of or witnesses himself. Soowon probably wasn't aware that his father killed Kashi, so he believed King Il just killed his father for no reason, or perhaps because he was afraid of a rival. This eventually leads to the start of the story.