It seems that a lack of willpower would start a vicious cycle. Failing once might give the sovereign more trepidation the second time around, making it easier to fail. And so on. The reputation of the king would suffer publicly. I doubt such a ruler would last long on the throne with the strong-willed and power hungry about.
Of course, the reverse might also be true. A strong king, able to intimidate all law breakers would be perceived as unbreakable of spirit.