@Kaarme He's certainly a clever and active ruler. And he's an excellent battlefield tactician. I'm not sure he's actually a
good king, as in someone ruling in a way that's good for the country. For someone as clever as he is, he seems not to have any kind of apparatus for keeping the central government informed of what the hell is going on in the provinces, let alone the near abroad, resulting in his getting blindsided repeatedly by crises. Whatever his faults, King Il managed to last a fair number of years in the job without the kingdom falling or seeming to be in particularly grave danger; since Soo-Won took over, there have been at least two occasions that I recall, maybe three, where the kingdom would have fallen if a group of crazy, powerful independents hadn't happened to be on the scene. And generally, this has happened because of major problems building up that Soo-Won remained completely unaware of. Admittedly, some of his problems were festering since King Il's reign, but far from all.
Further, his expansionist policy strikes me as unsustainable, and one which he is engaging in less because it's inherently a good idea than because he is emotionally committed to his late father's policy being right. His rulership style seems to involve never telling anyone what's going on. The most loyal people end up largely directionless and confused, while the least loyal pursue their own agendas. Since he's so brilliant, all this often ends up with his people doing what he wants without having realized what it was that he wanted, which is fine as far as it goes, but it doesn't build up a government or set a direction. He is in some ways too fascinated by his own brilliance to do a good job; rather than the right thing, he always does the clever thing.
Finally, he has a major inherent weakness whatever his personal qualities simply because as a usurper he lacks legitimacy, and gives challengers the idea that if he can get away with it, why not them? That has already led to a major episode of treason which nearly toppled the kingdom.
So that brings us back to revenge. If we set revenge aside, as a matter of pure policy she should kill Soo-Won, and as quickly as possible. She is the rightful heir of the last legitimate king, and the kingdom won't be stable until she is on the throne and usurpation is seen to be punished. But revenge itself--seeing it as questionable, or something to be potentially set aside for the good of the state, is a very modern idea. Taking revenge for the killing of her father, and for regicide, would be seen as extremely legitimate--more than personal, for almost any non-modern setting it's practically a principle of statecraft. Failing to do it wouldn't be seen as benevolent, but rather as both weak and immoral--breaking faith with the dead and, more specifically, with your revered ancestors. What kind of filial piety is it not to exact retribution for your father's murder, just because you personally happen to like the guy? And because of the way Soo-Won has failed to organize the state around him, I'd say she could whack him and move into the power vacuum pretty easily. Everyone except that cunning advisor guy would come around pretty readily to seeing her as the one in charge--the legitimate heir, that most of them know and like, with the legendary dragons and the almost-legendary Thunder Beast, revered by most of the soldiers, backing her, and with nobody else that I know of having a particularly great claim. Next after her would be the leaders of the different elemental provinces and none of them would accept any of the others as the boss.
Overall, it seems very likely she would (and will) be a better ruler than Soo-Won. She is a team-builder. She inspires loyalty, and then lets her loyal people use their talents. She is focused on building rather than manipulation. She is neither weak like her father, nor dangerously aggressive like Soo-Won (not that Soo-Won has an aggressive personality, but his policies are aggressive and IMO very likely to be dangerous).
Of course it's all moot since he apparently is going to so-conveniently croak without the mangaka having to dirty the heroine's hands.