If Mage and Magician are valuable resources and manpower and that mage is really good enough to work for the royal, then Isn't it would be wasting for killing him? I mean maybe, the King thinks that he has other reserve mage or what? And if really killing is the most effective solution, why don't make it official work, like accuse him for some fake crime would be less damaging the reputation of the royal? Please correct me if you have a better idea.