Like the strong hints right now that he killed yona's mum, and the after effects of killing the priests is obviously that some kind of punishment will catch up with him, in turn driving his wife's mother to suicide. Not to mention the war atrocities he committed in that other kingdom which traumatised the now Queen and directly led to the. Succession fight. Like... Even with the extremist worship they had, that kind of brutality isn't what you want of a wise ruler, you don't want to be known as a ruler who took a sledgehammer to a wound that could be fixed with a scalpel.
But then yona's dad went too far the other way, both bc it seems he didn't want to repeat his brothers mistakes and bc he wasn't actually raised to be the ruler. And of course resentment and hatred of his brothers actions which escalated everything in ways no one ever wanted them to. Of course also effected by the fact no one but mundok blessed his leadership, and almost no one had any expectations for him. I wonder if he was purposely just trying to act as a placeholder while blindly believing his kid could fix it all. There's also the fact he could have known suwon would murder him too...
It's just sad all around. Like you can literally draw every modern event in Kouka back to Yuhom' s biggest acts of brutality and the domino effects of them. Even without the multiple povs we have as readers, a person thinking in their right mind should be able to tell that purging an entire group of people won't end up happily effecting your loved ones.