What is the theme in Guards! Guards! by Terry Pratchett?

The desire for power is a common theme throughout history and is easily identified. This theme takes a different turn when it turns out that the person's desire for power takes the form of a dragon out of his own imagination. Wonse had power. He had the ability to make commands and have them followed to a certain degree and enjoyed the fact that he was able to be in a position above Captain Vimes. He wanted power but didn't desire to be king himself. He talked about the fact that he wanted a king that would be easy to control. His desire for power took shape of the thing he was trying to use. The more he let it out and used it the more it grew in strength until it took over. Later on Wonse has a laugh that is described as being a mad laugh as if he had lost his mind.