Should we blame the misfortunes that occur in Oedipus Rex on Laius, Jocasta, Oedipus, the shepherd who saved Oedipus as a child, the gods, or fate?
If anyone gets the blame here, I think it should be the gods who determined Oedipus fate to begin with. Though Oedipus may deserve some blame for rashly killing the man who blocked his way on the road (his father), he was only acting out in response to the affliction meted out to him at birth.
I think it's an exploration of fate, and what happens when we think we know what fate is going to do to us. This is a theme that is repeated over and over in literature. You can try to prevent something from happening, but if it is going to happen that it will happen and there's nothing you can do. This is one of the original versions!
I really think this one is due more to fate than to anything else. Sure, Oedipus could have refrained from killing Laius. But this was a time and place where men were trained to defend their "honor" and their pride. So it does not seem that a Greek audience would really have blamed him for killing Laius.
So you have Oedipus who has done two things that were seemingly not really blameworthy (killing a random person who offended him and marrying Jocasta). It is only fate that makes these things "sins" -- it is no individual's fault that the person Oedipus killed just happened to be his father or that the woman he married just happened to be his mother. It's just fate.
The misfortunes in Oedipus Rex happen for a combining of reasons. King Laius could have killed the infant, Oedipus, but did not. Jocasta could have killed Oedipus but did not.
The shepherd could have left the child to die or be eaten by wild animals. While the gods and fate have a part in the prophecy of Oedipus growing up to kill his father and marry his mother, Oedipus is ultimately the blame. He killed his father because of a lack of self control. He allowed rage to enter his reasoning. He killed his father out of anger. He did not use restraint.
Truly, Oedipus is responsible for his actions as a grown man. He did not have to murder anyone. Had he used self-control, the murder would not have happened. King Laius would have lived and Oedipus could not have married his mother. Oedipus made a decision to murder another man, a man that just so happened to be his father.
Although the reader sympathizes with Oedipus, he is still to be held responsible for his father's death. Obviously, the gods hold Oedipus responsible by sending the plague on Thebes.