Beati Pauperes Spiritu...

Oedipus cursing his son, Polynices, Fussli Johann Heinrich, 1786.

Since the founding of religion, the gods have sought to test those with power. Jocasta was sent by Apollo to do just that: to test Oedipus' faith and conviction.
Throughout the play, Jocasta tested the beliefs of those around her by feigning disbelief in the gods herself. Though she put up this false front, she did keep her faith. At the beginning of the scene wherein a messenger relayed word of Polybus' death and Oedipus' ascension to the throne of Corinth, we see Jocasta praying - not the act of a disbeliever at all! Again, in the same Episode, Jocasta carries garlands and incense to the altar and tries to appeal to Apollo to purify the city of Thebes.

While her faith remains strong, her task from the gods is to test the faiths of others, especially that of Oedipus, which she does. After accusing Creon of conspiracy and treason, Oedipus relates to Jocasta the details of his meeting with Tiresius. Jocasta proceeds to plant doubts of the gods by telling Oedipus the story of the Delphian Oracle and the circumstances surrounding Laius' death. Again, after Polybus' death, she excitedly tells Oedipus that his prophecy was obviously untrue, though it wasn't, and by doing so she attempts to hint that the oracles - and thus the gods - are false. By planting this doubt in Oedipus' mind, the gods are able to test his faith, and his ruling power, through Jocasta - a test which he fails until it is too late.
Why, then, would Jocasta be forced to perform such a dubious task for the gods? It is because she tried to avoid an earlier prophecy. By tying her child's feet together and casting him out, she attempted to defeat the gods, and this of course angered them. Her punishment, then, was to test the beliefs of the very child she cast out.

Oedipus Rex was Sophocles' attempt to show the Greeks that they could not avoid the dealings of the gods, or they may be forced to conspire against the very people they should love most. Jocasta was, in this way, a victim. Though it was by her own doing that this penalty was cast upon her, it was not something she was happy to do, which becomes apparent when she realizes the truth in her earlier prophecy. It is at this moment that she becomes aware of her punishment, and in desperation kills herself. Oedipus Rex is a true tragedy in the sense that fate and Oedipus' own tragic flaw (his hotheadedness) combine to bring about his downfall. What makes it that much more tragic, however, is that the events were, for the most part, out of Oedipus' control. The chain of events was set in motion by his own mother. He was forced to leave Corinth (thus trying to avoid his own prophecy), afraid he may bring harm to the persons whom he loved the most. Scared and angry at the curse he carried upon his back, who can blame him for being hot tempered? It is Jocasta, his own mother and wife, that brings the story full circle by again cursing him, perpetuating a disbelief in the gods.

Jocasta is a victim in Oedipus Rex, but not as much as she is a catalyst for Oedipus' own victimization. She keeps her faith throughout and tries to relieve Oedipus of his. Because of this, readers may in turn pity her and loathe her. But the gods tested the king of Thebes through her - the main goal of the play - and both he and she failed.