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.