The Oresteia is like a nightmare that keeps repeating, but with slight differences each with each repetition. Because successive generations commit similar crimes over and over, it seems that the curse of the house of Atreus is almost written in their DNA. The curse of Tantalus, Pelops, and Atreus is one of child murder and sacrifce—a primal taboo on what must have been at one time an actual practice. Orestes reverses the curse, as it were, to murder his mother and her consort, Aegisthus. Arguably, either Agamemnon's sacrifice of Ihpigeneia could be the original sin, the origin of evil (arche kakon); but the primal crime of Tantalus seems to be the origin of the guilt. The word aitia (etiology/origin) also carries the implication of guilt in ancient Greek. It starts to make more sense when we think about the role that prophecy plays in the Greek tragedy. Tieresias, Cassandra, et al. do not foretell the future so much as interpret the past in ways that the characters are incapable of doing. Cassandra in particular, when she comes to the home of Agamemnon, has a vision not only of Agamemnon's death, Orestes' vengeance, but also of the ancient crimes of Pelops, et al. The prophet knows before others can realize it how the past continually shapes the present moment. The failure or delay of such knowledge by Oedipus, for example, becomes his hamartia, or mistake, which leads to his downfall.
