1) It's Oedipus's fault:

Aristotle, in Poetics, written in the next century after Oedipus was performed, defines tragedy. Aristotle considered Oedipus as the best representative of the tragic hero, a "man in great repute and good fortune,"not exceptional for excellence and justice, who undergoes a change to misfortune not on account of baseness or villainy but on account of some error." Hamartia, is now believed to mean an intellectual mistake, not a moral error or "flaw" of character.

2) Oedipus must be punished to uphold the moral order of the universe:

Oedipus is guilty of terrible crimes and the gods cause his downfall to illustrate their enforcement of moral laws.

3) The gods don't care about human suffering:

Though Oedipus's acts violate the laws of gods and men, he is morally and legally innocent. His downfall illustrates a world order whose workings do not square with human conceptions of justice. There is a gap between the remote gods and any sense of meaningful human suffering.

Sophocles places us in a tragic universe where we have to ask whether the horrible suffering we witness is all due to design or to chance,whether our lives are random or entirely determined. If everything is by accident then life seems absurd. If it is all by design, then the gods seem cruel or unjust and life is hell.