Aye, Hak does mention that there was only one person whom he knew was stronger than him and whom he couldn't beat – but this fight was either during one of the tournaments, or maybe when Hyoo-ri was teaching Soo-Won and Hak may have challenged him. I'm gonna go with the tournament.
Also this match between them was when Kouka was "peaceful". No wars, no battles. And when he was still young. It's different now. Hak has battle experience and he's grown. His body and strength have also grown. Not to say that Hyoo-ri hasn't been training, but I do wonder how many people he's killed since that night of Il's death. True that we haven't seen him shadow Soo-Won during his escapades to the Tribe cities. He probably knew Hyoo-ri may end up causing a ruckus while his intention was to remain incognito and so asked him to stand down and remain in the castle.
But...
Is Hyoo-ri even allowed to kill people who haven't "attacked" first?
Hyoo-ri knows Hak and the position he's in, but wouldn't he also require permission to kill him off when his first option would be and should be to capture him as a prisoner?
In that case, why can't Hak assume he'd be captured? Sure, it was a risky bet indeed on Hak's part.
Hak didn't attack Soo-Won when he was close to him, so it doesn't warrant a kill but capture (yes, it's true he was warned, but then he should have been attacked right there on the stairs no, but that meant putting the weak Soo-Won in danger). However, they're on castle grounds which means it may cause distress, plus there are delegates from another country in the castle, and you wouldn't want to cause any further ruckus than what's already brewed with Lan Tan; so I assume Hyoo-ri attacked Hak when noone was around to avoid being seen, but probably to knock him out and capture as prisoner for futher decision. And he knows Hak is strong, so a quick strong blow would be needed. Yet Hak avoids that and managed to defend himself, so Hyoo-ri needs to get more intense. However, Hyoo-ri's not a decision maker, he's only serving as a guard. And he knows Hak is in the castle on special terms. So he would need to capture him.
Meanwhile, Hak knew Hyoo-ri won't speak. So if he's captured, then someone with power will come like Joo-Doh or Kye-Sook, because Hak said something he should not have known (King's condition). And so, the higher authorities would need to confirm/ascertain that it is indeed the case and then pass judgement. When Hyoo-ri went to get Kye-Sook, Min-Soo was probably around.
And then of course the nuance of the looks are enough. Judgement is passed. Min-Soo has revealed his betrayal. Yona appears and manages to save her beloved because Hak can't get killed just yet.
Hmm. If he did die, ooh, Yona-chan would have gone raging mad and unleashed the dragon power? Or she would have slumped like she had when Il had died?
