@ninjadork
@Meridis
@Austriacus
Giving the king's line a bit more thought, "idealism" is a poor word choice on the writer's or scanlator's behalf.
"Optimism" would've been better and true. Using that instead of "idealism" wouldn't have implied "having ideals is foolish."
The princess was correct in wanting confirmation of whether Fulio had sided with demons, but it's naive/idealistic suggesting they ask the very person under suspicion of being an enemy if they are an enemy. If your only info is that someone in
town the kingdom is being visited by
terrorists/jihadist/etc your enemy at war, it's a bad idea to lose the tactical advantage of having the first move.
Now instead, the king chose an infinitely worse alternative:
let the retard handle it. Send the mentally-disabled guy with a sword to tame an already domesticated bear that's keeping the hornets' nest in check. Even if the king decided they were an enemy, sending anyone else had the possibility of listening during the confrontation.