If you kill someone, whether you feel it's justified or not, it's murder. And who's to say the Yona's dad really did kill Soo-won's dad...or that he didn't have a good reason? Maybe Soo-won's dad was plotting to kill Yona. Maybe he was responsible for the "bandit attack" that killed Yona's mom. So, then by your reasoning, him killing Soo-won's dad would be justified and not murder.
Sorry...wrong chat to discuss Yona, but couldn't resist :)
I personally believe that murder is never justified, but even if you are one to support the death penalty, Soo-won had no evidence that Yona's father killed his. (I even went back to the beginning to confirm this.)
Being such a peaceful man, it was most likely not her father. Soo-Won is acting out of emotion, ruling out the most logical explanation to find a scapegoat for his pain. It's uncharacteristic sure, but this kind of illogical grudge started when he was a child so it makes sense that he clung to the mindset. Because he's producing better results than Yona's father ever did, he probably rationalizes the murder by saying it was for the welfare of the country instead of just a petty revenge.
Yeah right avenging his father by killing the king, betraying his friends and attempting to kill them. A cycle of never ending hate to each party. Totally right! How come you think killing is okay if it's for the sake of revenge? I don't really care much of the King but he ordered to kill his friends too. I understand that he wants revenge but saying Soo-won didn't do anything wrong makes me want to question your principles. They're both wrong admit it. Except Yona and Hak who are innocent and just trying to survive themselves.

Ew it's Soo-Won