Trouble, Tears, and Tragedy
As Aristotle said, a tragic hero is essentially a good character with a flaw. The flaw will ultimately bring about misfortune and a sad ending to the tragedy, which makes the audience sympathize with the hero and fear the book's impending conclusion.
Take Oedipus Rex by Sophocles, for example. Oedipus is the tragic hero, and he is prophetically fated to kill his father and marry his mother. Here's where the flaw comes in. It might seem that his flaw was partly in struggling to avoid fulfilling the prophecy (and so making it reality), but considering that he still failed to escape the prediction despite all his efforts, it seems that it was absolutely impossible to prevent it from coming true.
Another more reasonable suggestion for this tragic hero's flaw is that he was too hasty in making decisions. Upon receiving word that Apollo was punishing Thebes for harboring Laios' murderer, he immediately called the people together and announced a dark punishment for the offender. He went so far as to say that even if the murderer were a guest in the king's own palace, justice would be carried out. As the king, his intentions were solidly good; he wished to help his city by ridding it of the plague. That would be accomplished by avenging Laios' death, and he made his proclamation in order to do so. He did what any good king would do, but his words later condemned him.
It was also haste and, more specifically, a quick temper that drove Oedipus to murder Laios. He was angered by the Theban king's haughty actions, and his temper flared. He killed Laios, who was actually his father, therefore making the Delphic prophecy true. The fact that this awful deed is revealed bit by tiny bit makes Oedipus Rex attention-grabbing and suspenseful, and it also adds to the horror at the end when the terrible truth is confirmed.
Oedipus was a heroic man and a great king. He solved the Sphinx's riddle and freed Thebes from her death-wielding claws. His flaw, however, was in being too quick to act. Had he not killed Laios, he would not have married his mother, and the story would not have been a tragedy. On second thought, it probably wouldn't have been written at all. The Greek gods are, however, mysterious and often times cruel. If Oedipus hadn't killed Laios at that particular moment, perhaps he would have met him at a later time, with the end result always holding true to the prophecy. There are always the "what-if" endings to a story. In this case, Sophocles chose the ending he did, and so this book is labeled a tragedy . . . starring Oedipus as the tragic hero.
~Fine ~