@imtaciturn Normally I would agree with you. Looking at this Demon Queen though...
She is so weak that the other "heroes" immediately pointed that out in chapter 1.
There is no direct conflict between humans and demons (Chapter 3)
She is placed in enemy territory by the hero for over a month and nothing happened.
Now as far as conquering the world goes/'world peace'. The King might as well "announce" that the Demon King is dead and the demon territory is now his (unless he is genocidal enough to provoke the hero)
As far as marrying the hero for prestige... he could literally just get a 'scapegoat'. Or get a new hero and have his daughter marry him, whichever works. I don't know why these royalty keep thinking going back on/provoking that 'certified strongest existence on the planet' for whatever reason is ever a good idea. It is not excusable
especially if they know he is the strongest (and placed him in that position with the knowledge)
I mean... that one other King did it to that one other hero and his whole kingdom went kaput. (different series).
Seriously, as Kings you'd expect them to have a minimum level of self preservation knowledge.