There's no guarantee that his skill doesn't lie, so far he did preceding attacks or actions, not results of their supposed bad intentions.
It would be an interesting take if whatever grant him that particular skill, did that to set him up against the kingdom, like IF the Demon Lord influence it.
Like I mentioned before, they summoned an overpowered being that surpasses even the royal knights captain in skills he cannot learn as they're only granted to summoned heroes.
He mentioned that they have to summon heroes each 100 years, the records aren't detailed, they don't have a list of skills previous heroes possessed and his skill looks like a literal brainwashing skill.
I don't say their hatred to "heroes" is justified, but...
I'm wondering why the princess is racist to anyone not human, maybe it's not that unjustified?