The term protagonist can be applied to anybody, whether it is a person in this case Orestes, or an institution, however, the motive stays the same. In made up stories it is being described as a destructive force and the fight especially in greek tragedies and continuously the fight must go on until the hero thrashes the villain that is antigonist. The word protagonist is simply an antonymous term for the antagonist and the hero or the protagonist acts opposite of what antagonist acts. There is no definition that can easily define the destructive force or the constructive force, but as long as the story progresses the heroic and villainous traits of characters evolve and it gets devided into protagonist and antagonist. Sometimes it can happen that antagonist and protagonist, both qualities stay in the same character and is decided only by acts at some point of the play which he does and where he belongs.