Wait... my question is: how did those two end up together?
But with them ending up together plus Yu-Hon killing the priests and stuff I could actually see Ill killing his brother, if something pushed him over the edge. I'm still kinda doubtful if he actually killed his brother, being pacifist and all that, but everyone has a limit and if Kashi died because of Yu-Hon, I could see him doing it.
I can't really remember the part where Yona met the priest, but maybe she being Kashi's daughter also made him help her out (other than being the reincarnation of King Hiryuu.)
So we have:
King Hiryuu having descendants that have a 50% change of dying before turning 30 (or around that age) -> Yo-Hi (Soo-Won's mom, I'm not that good with names, so I'm probably butchering them) meets Yu-Hon, secret gets outs, they get engaged, priests realize she is a descendant of King Hiryuu, Yu-Hon sends them to their death -> Kashi and the priest (I forgot his name) manage to escape, but Kashi gets some burns during the escape (probably there, might have been elsewhere though) -> Kashi somehow meets Ill and they fall in love -> Something probably happens and Ill kills(?) Yu-Hon, taking his place as the next in line to be king -> Soo-Won gets to know about his father's death -> He kills, the now king, Ill -> Yona story's begins.
So if you want someone to blame, blame King Hiryuu, the dragon that wanted to be on Earth. Still, a lot of what happened could have been avoided if people didn't send heads flying when they got angry.