I don't think that Soo-won took the throne just to give it over to Yona. At the same time, I'm second-guessing my earlier thoughts that he tried to kill Yona, too. In the beginning chapters, it looked like he just planned just to kill Il and to leave Yona alone. Then when Yona witnessed it, he tried to kill the witnesses. What if he sent his head bodyguard after Yona that night to make sure she escaped though? If that guy is so strong, it doesn't make sense that a pampered princess got away even with help. And the one person that did get sent after them was a complete joke.