In a traditional story, the villain is a catalyst for story. Usually, the events of the world would follow a destined course of action, but the villain decides that they want more, or they simply want the world to take a different path. Once one person steps out of the shadows to change the course of destiny; another person inevitably steps up to stop them. In more complex stories, a second party might not seek to stop the changes in progress; instead they may seek to create further changes of their own.
This great how to book continues the series of tips and tricks, this time around Avalon Games is looking to improve your villains.