Up to date with the manga scans and following some people on tumblr who've done summaries of the chapters for a while now, between both I think I've actually read/read of each chapter!
And yeah, Soo Won's motivations remain the one big mystery in the story, I feel like that's going to be the bit that joins everything back together in the end so we've got a while to wait....
Ah yeah, I've always wondered if his brother was planning a secret attack or something and King Il killed him to prevent it (since, Kouka just isn't prepared enough to fight off a huge enemy) but this is all waaaaaay speculation on my part.
Saw that you also read the Yona manga, what did you see in there that seemed to explain King Il killing his brother? I can't remember anything off the top of my head....