2 thoughts on this:
First, the "Villainess" trope for Korea & Japan originates from otome games, where the Villainess is more of a story role than a personality type. While it's generally ideal if the 2 overlap, it's hard to root for a genuinely evil character. There are some manga out there that manage to pull it off, but it's usually a "reality check" storyline where the villainess is strict but smart.
Second, I have to respect the King's decision here. The ability to force people to tell the truth is invaluable, assuming the person with the ability isn't an idiot (which the MC here isn't). The use cases in diplomacy and counter-intelligence are numerous, with the simple threat of their use doing more work than direct application.