Yuhon found out about Yonhi's 'curse', and went crazy trying to look for a cure. Not sure why he decapitated all those Xing POWs, but perhaps they did something to Yonhi. He would have gotten so desperate and angry at the gods (think Zeno when his wife was dying) that he chased out all the priests (like Iksoo) and tried to destroy the temple. Il, who's shown here to be very religious, might have been really mad about that and stabbed him to prevent him from harming Yona, whom Il believed to be the reincarnation of the king that was the reason for Yuhon's wife's and son's 'cursed' blood. OR maybe Yuhon knew about Yona already and somehow Yona's mom died protecting her from him, so Il stabbed Yuhon in retaliation.