Why does Oedipus blind himself in Oedipus Rex?
After he accuses Creon of trying to take his throne, and the seer Teiresias of being blind to the truth in Scene I--"Why, he is no more clairvoyant than I am!"--Oedipus comes to realize that in his hubris, he has ironically placed his maledictions upon himself rather than on the seer or Creon. Before he learns the truth, Oedipus articulates this curse:
May I never see the day! Never!
Rather let me vanish from the race of men'
Than know the abomination destined me!
Then, in Scene 3, Oedipus learns the terrible truth and exclaims,
O Light, may I look on you for the last time!
Oedipus, damned in his birth, in his marriage damned,
Damned in his blood he shed with his own hand!
Truly, in this drama the words of a messenger--"The greatest griefs are those we cause ourselves"--explains the absolute despair of Oedipus and his "madness" that "[L]eaped on" him and made him punish himself. In Stophe 2, he explains why he blinds himself,
How could I bear to see
When all my sight was horror everywhere?
Thus, Oedipus blinds himself in order to punish himself, and to prevent himself from ever seeing Jocasta and his children against whom he has sinned. That is, since he has been metaphorically blind for so long, he physically blinds himself in a gesture symbolic of his ruin. This self-punishment also demonstrates the act of justice that Oedipus places upon himself, for he believes his suffering is deserved since in his hubris he has been convinced that he could end the plague of Thebes.