Soo-won's father was bloodthirsty. He was planning on becoming king (killing king and Yona) and taking over all the countries. The king killed him to prevent this coup and war. The King thought Soo-won was a lot like his father and didn't want him to have too much power. Soo-won was angry at the king for killing his father and felt the king was too weak and it was hurting the kingdom. Yona has realized that her father's kindness had allowed his people to suffer at the hands of pirates and criminals and she understands why Soo-won took over as king.
i think it's because he knew that soo won was evil and that yon awas the king hiryuu
I think that Yana's father was hiding something even if he disliked his bloodthirsty brother I don't really understand why ke disliked so much Soo Won I think there is something else and why did he let his country go away like this. He appears scary two or three times already and each time in weird situation I think he was planning something to run his own country in a way or an other.
Yes, I think there is something behind the entire thing. What unsettles me however is that Su won idolizes his father and believed his father was 'right' and that he was a great general when we've been shone that he was quite bloodthirsty too and he even justified his father's actions of hanging enemy soldiers head at the wall as what hapoens in war, however, one of the foreign princesses told her side of the story and in it, she said Su Won's fsther had promised to release them (if im remembering this correctly.)
Now I'm wondering, what if Su Won and King IL had actually planned this entire thing? King IL chose to have Su Won kill him and chase Yona out so that she would grow up into a great ruler, whil Su Won sacrficed his freedom and acted as the villian? (XD just crazy rambling lol)
I agree it's just crazy rambling (but so much fun!!). To be honest the bloodthirsty personality is said by the princess but she was a traumatized child and he was a general in a war(not to justify his action). So maybe, I insist on maybe, he was a violent man on a battlefield but a good ruler for his country he was just lacking of diplomacy. Whereas Il was a bad ruler said to be too kind, I don't think so. I'm looking forward the new arc, maybe we'll have some answers.

I'm still waiting to find out the reason why Yona's father hated/disliked Soo-won, and whether Yona's father really killed Soo-won's dad (and if so, why).