because what if you are planning a crime or suddenly "snap" one day and take your vengeance out on other people?

Baking that assumption into society suggests there are deeper problems, but let's ignore that. Why can't the king's subjects defend themselves against sudden-onset brain damage, just like they do invaders?

Benign monarchy has its positives (like having somebody specific to blame, instead of the mutual mistrust that develops in a dissatisfied democracy), but not all historical kings could or would use mercenaries for defense. Those that did sometimes regretted it.