They call it "King's disease" or "royal madness", what paved the way to American democracy

The most relatable way to understand it is to compare it to playing a video game for an extended period of time with a maxed out character. Or some sort of "sim/rpg" as a moderator or programmer.

You end up thinking everything is fake. Everything. Like God himself is mocking you.

You say something. It happens. You want something (Your staff knows your tendencies better than you at this point) it seems to just magically and randomly happen. This is what led to the madness. Psychologically it is an (obviously) rare form of existential crisis where the afflicted individual slowly becomes unable to distinguish reality from fiction, but in a way where they simply do not believe they exist nor do any of the people around them. It is often brought on from isolation (anyone/everyone is trying to kill you so they can become king) combined with the simple reality that there is no challenge or struggle or basically anything remotely interesting you yourself don't declare to occur. To an intellectual it suggests reality is no longer real and you are trapped in some sort of simulation.

Led many great men to their deaths back in the day. And worse, entire populations. Hence the phrase "the mad king"

This is where they decided democracy would be the best substitute. Even the most powerful man in the world has enemies. It's ironic. Those who would kill him, end up saving him. Very surreal. But, this is the world we live in after all.