Before having an answer to this question, we first have to analyze a little of Aristotle's concept of tragedy. According to him, tragedy must be about reversal of fortune of the hero and the reversal should be from prosperity to adversity. This reversal must be an outcome of character's own error of judgment. It must not be only because of fate and character's individual participation should also be there. Hero of a tragedy must be a noble man and he must have all the good qualities that a good man can have. But at the same time there must be some flaw in his character and the reversal must be resulted in consequence of that flaw. This flaw is called a tragic flaw and Aristotle puts it as "Hammartia". For example, if we analyze "Oedipus Rex", we will see that he is a good and noble king. He is fair and just to his people. He has got good leadership qualities as well. But, nonetheless, at another level, he sometimes becomes proud of his acts and shows his intelligence in a pompous way. Similarly, he is too quick in formulating his opinion about others. Moreover, he is a bit too inquisitive. And above all, he is short tempered. These are the flaws due to which Oedipus killed his father unknowingly. I.e. Fulfils the oracle.
