The evil within one person, in ways, is like a viral disease. It can be within one and after some time infect another, and it maintains doing this regularly until the last evil person dies without transferring to another one who cannot avoid the evil. A great example comes from movies, it's the hero's job to get rid of evil, and there has been a movie or play where in fact the hero destroys bad and then become bad himself. In Hamlet, from William Shakespeare, Prince Hamlet takes on the main character and as well the hero of the story. Hamlet decides to get rid of the relationship of Claudius, the new king/evil person, who wiped out the old king, became the new ruler himself, and committed Gertrude the mom of Prince Hamlet. The visitors can see Hamlet's action changing through the play from good to bad.