as much as i agree with the consent that everynoe but the farther are idiots, you need to look at it from their prespective
for them, every single time a demon king was born,(and a hero in return and vice versa) the world was engulfed in war not long after, to them its a force of nature, a rule set in stone eons past.
Something that can NOT be changed, furthermote the demon king is a calamity, rallying demons behind themself and causing devestations as they go, the hero is a powerfull being strong enough to topple nations on their own. Now, a person comes along, who takes the Hero, AND the demon king in and raises them to be his children(he means it earnestly, no deceptions here), the first reaction you have is to either assume he dosnt know they are hero and demon king(unlikely) or to assume he is "brainwashing" them to do his bidding, furthermote, its probbly unthinkable that the demon king can be contained by just being raised nicely, and that someday the demon king will release her armys on the world, but without a prepared hero it woudl be devestation.
The Kingdom, and everyone else have a "point" in trying to take the hero back, its stupid, and at this point is a self fulfilling prohecy, but they have a point in the logic of the world
@fgosegul they dont want a war, they think war is inevetible, because well to them in their world with their logic it is