Demons, like the undead, are not human. They represent a different internal state which human beings must confront within themselves and others. That state is rampant emotionalism, or being run by our feelings, being lustful, angry, hysterical, paranoid, fearful, filled with hate, being greedy: states of mind which detract from and interfere with more benign human functioning. So to defeat a demon means to symbolically take control of your internal emotions, and not let them run your life.
Read previous entries in this series about symbolism: of Ruins, of the Undead, of Deserts.