Why has Shakespeare written Macbeth?

Shakespeare wrote Macbeth specifically for King James I, who was a huge supporter of the theater. The problem was that the king was easily bored; long, drawn-out plots with minimal action allegedly caused King James I to fall asleep in the theater itself. Therefore, Shakespeare produced Macbeth, which is by far the bard's shortest and most violent play ever (also, Macbeth included witchcraft because the king was fascinated by the concept), in order to not only please the king but keep him awake during the play.