Many sci-fi/fantasy stories have main characters that gain access to secret abilities or a device which is usually considered dark/evil/taboo. The main characters then try to use these powers/devices for good although their friends warn against it, and for a while it seems to all work out for the best. But in the end, the naysaying friends are proved right, either b/c the main character temporarily went evil, or b/c he accidentally lost control and hurt a friend or bystander. Thus we readers learn that evil can't be used for good. But why not? Can't there ever be a time when this rule doesn't apply? I.E. Why can't a good witch/wizard use dark magic to take out bad witches or wizards?