Heavy is the head that wears the crown, and occasionally it leads to madness. The trope of the mad monarch is a common one in worldbuilding, likely because it can lead to so many additional (and exciting) associated elements. With mad kings and queens come coup attempts, assassinations, beheadings, wars, strife, and all other manner of upheaval. Kings and Queens can wield immense power, and a world can change drastically at their whim. The addition of a mad leader in your own fictional world may help provide just that touch of chaos your setting, story, or history needs.

Do not limit yourself to thinking about monarchies. Also consider other leaders as well.