Sooooo I'm hitting a ball in the dark but here is my guess. Somehow Ke-sook guy had a hand in Su-wons dad's death. His clan convinced a young Su-won that King-IL had killed him so he would want revenge. They could also manipulate Su-won further once revenge was taken. Yona has somehow started piecing this together and THAT is why she is going along with this bull $#%!. Yona and her group BELONG in the castle it's Su-won and his army of traitors that don't belong. I trust you Yona .... don't make me regret that. ┑( ̄Д  ̄)┍
Show me the BABIES! Σ(っ°Д °;)っ
It ended bestie Σ(  ̄□ ̄||)