Aristotle, "the last man to know everything," had insights into the art of writing tragedies. He wrote out a list of characteristics that a play must have to be considered a tragedy. The story of Oedipus Rex has all of those characteristics and would be considered by Aristorle himself to be a tragedy.
The first of Aristotle's requirements said, "The tragic hero is a man of noble stature. He has a greatness about him." Oedipus, the hero in Oedipus Rex, was the king of Thebes and saved the city-state from the sphinx by answering its complicating riddle. The nobility of Oedipus never seems to be a question to any of the people throughout the story. Not only did he become the king, but he was actually the son of the king, so he actually had nobility in his blood.
Aristotle's second requirement was that the tragic hero must not be perfect by any means and may commit a crime, fully in belief that he is doing it for good cause.
The hero then must be persecute for his actions because they are guilty despite any good intentions. Oedipus killed his father Laios not knowing that he was his father of that he was the king. Oedipus did not know of his crime. He simply thought he was defending himself.
The third requirement said, "The hero's downfall, therefore, is his own fault, the result of his own free choice-- not the result of pure accident or someone else's villainy or some overriding malignant fate." Oedipus killed his father in his own free will. He was the lone person at fault.
The fourth requirement is that the hero's punishment is not deserved totally. The punishment is too severe for the crime committed. Oedipus killed Laios, but he saved...