The Oresteia by Aeschylus is about the cyclical nature of Bloodshed, in order to stop it, the law must change from the barbaric eye for an eye to a more civilized democratic system.
Snakes are a common element used throughout the trilogy. They often represent scheming, intelligence, deception, and strangulation. The transformation of the snakes from the first to last book covers illustrate this change.
The first book, Agamemenon, sets up the cycle with family members killing each other, thus more family must kill each other to avenge the death of their kin, creating a continuous cycle.
The Libation bearers continues the family circle of bloodshed. All of the killing has become chaotic and confusing and the characters consider either breaking free from the cycle or continuing the violence.
The final book of the trilogy, The Eumenides, brings relief to this process as the gods help Orestes by setting up a democratic society. Old laws and tradition give way to a more civilized way of life. Each crime, now, starts and ends with that person.