king il is a very different character than we first see him portrayed. At first glance, we saw what looked to be a loving father, and a very kind man. He was a ruler who looked to be very weak and a coward, and a advocate for peace. And then through Hak's prospective we saw that he wasn't a coward, as he wasn't afraid to hurt himself to stop violence. So all in all, up to this point I personally had a pretty good impression of him. But now we see that he is religion obsessed, to the point he wouldn't even be a capable king. And as we saw, he automatically jumped on the notion that Yu-Hon had killed Kashi. and I agree with
@SylverDrawer that Yu-Hon does not kill without a greater purpose and is not afraid to admit when he has killed someone. He isn't a coward. Not to mention the fact he saw how much Kashi meant to Yonngi-hi and would not take away more people from her, Because it would be impossible for him to overlook the fact that when he had killed all the priests that it had really hurt her. King il had probably harbored a grudge since all the priests were killed, So he saw this as a perfect excuse for revenge. Just pass on the revenge plate to the sociopath 9 year old, who in turn killed king il. Lets hope our Yona can break this horrid chain with her distaste for revenge.