Orestes is a literary tool designed to express Sartre's philosophy of freedom. The stages of Orestes's development in the play mirror the necessary stages one must undergo in finding one's freedom. At first, we find Orestes struggling with the common notion of freedom, the idea that one is free if one has no attachments, no commitments, and is well off. This is certainly a kind of freedom, but it is not the true freedom that Sartre wants to make us aware of. It is a "freedom from" something, in that Orestes is not forced to do anything. He is free from persecution, from having to find a job, from any political duties, and from acting by any religious or moral rules. But Orestes rightly feels that in being free from all these things, he is not really free. He feels that nothing is really his: there is no city, community, or family that he can call his own. Furthermore, he has no reason to do anything. He is free from all responsibility.
As the action of the play develops, Orestes understands a different freedom that might be termed a "freedom to." He learns that freedom is not something material. Having money, an education, and slaves does not provide for the most important sort of freedom. When Orestes asks Jupiter to show him a sign, Jupiter obliges. In seeing this sign, however, Orestes realizes that he does not need to act on it: he is free from the power of the gods and from the control of all moral systems. Orestes understands that he is free to kill the tyrants who rule Argos and that if he does not kill them, then this can only be his own choice. The gods may have ordered him to leave, but he is free to interpret their sign as he chooses: he may decide that it means he should leave, but he can also decide to stay.
Jupiter wants to cause Orestes to take action, to leave Argos, by giving him a sign. But Orestes's realization that he is free means precisely that a sign cannot cause him to do anything. Having seen the sign, he can choose how to respond to it. His freedom is the freedom to act, a positive freedom. In killing Aegistheus and Clytemnestra, Orestes creates new values for himself. For example, he commits himself to the idea that freeing people is more important than abstaining from murder. The result is that Orestes cannot feel guilt: his freedom is both the freedom to act and the freedom to interpret the world. Since he has interpreted the world in such a way that killing the king and queen was right, he views his action as having been right. Orestes's development is a movement from the common sense notion of freedom to a deeper understanding of the concept.
Jupiter is Orestes's most obvious opponent. He creates the moral systems intended to order human action. By inspiring fear in his subjects and by showing them signs, Jupiter hopes to force them to act in the way he wants. There is only one flaw in this design: Jupiter can't actually force anyone to do anything. Only human beings can decide whether or not they will follow Jupiter's wishes; his goal, then, is to make sure that they do not realize that they have this freedom to decide for themselves. Since Orestes is the only person in the play who understands what it means to be free, i.e., to have the ability to choose for oneself instead of doing what one is told, he is the clearest threat to Jupiter's reign. Jupiter's character does not develop in the course of the play. Because he is a god rather than a human being, he does not have the ability to change over time. Jupiter is not really a person: he is an image that people keep in their minds. He must always present the same image of himself to human beings: the image of a supreme judge to be feared and obeyed.
The character of Jupiter stands for all systems of political or moral authority. By forcing rules for action onto people, these systems attempt to deprive human beings of their power to act freely. Sartre cleverly interprets farce and melodrama into his presentation of Jupiter. Jupiter's tricks of making flies fall down and his ability to move stones appear fairly silly: he raises his arm and speaks nonsense. Sartre wants us to realize that institutions that seek to limit human freedom are only images that maintain their power because human beings believe in them. What lies behind the image isn't power at all; the power is contained in the value we ourselves assign to the image. The comical way in which Jupiter carries out his magic tricks and the melodrama with which he gives his orders show us one thing: what lies behind all moral domination is farce.
Electra turns out to be Orestes's foil. Her development closely parallels his, but in the opposite direction. When they first meet, Orestes has no intention of doing anything in Argos; he plans to leave and wants to take her with him. Electra, on the other hand, has lived for years with the belief that she has a destiny that she must fulfill: she must avenge her father's death. This acceptance of her destiny is, in the end, her downfall. Her destiny is a fantasy. It gives her meaning and purpose in life. Consequently, when her destiny is carried out, Electra has nothing to live for. Unlike Orestes, she cannot define herself through her action because she only knows how to define herself through her destiny—an outside force telling her what to do. Revenge was the one thing that made her life meaningful, so once revenge is no longer something she can look forward to, her life loses meaning.
Before the murder, Electra is confident that to carry it out is the right thing to do. Revenge is what she values most in life. With the action completed, however, Electra sinks into cowardice. She lacks the courage of her earlier beliefs since, paradoxically, what gave those beliefs power was the fact that she had not acted on them. Once her hatred is gone and her life has no meaning, her value system dissolves. Since revenge is no longer a key value for her, she is forced to turn to others for values, and she realizes that others would judge her as a killer. Because Electra no longer has a value system of her own based on which she can claim she did the right thing, she must accept the external value system that tells her that she did the wrong thing. Electra therefore repents of the murder, leaving Orestes alone in the certainty that it was the right action.
Aegistheus is Jupiter's agent among human beings. His goal is to impose order on human societies. To impose this order, Aegistheus comes up with a clever way of blinding his subjects to the fact that they are free: he tells them that they are guilty of Agamemnon's death along with him and that they must atone for their sins. In his city full of remorse, everything is peaceful. No one will step out of line because everyone fears the judgment of others. Everyone repents of every sin they've ever carried out; regardless of how they themselves may feel about their sin, they accept the judgment of others, especially of Aegistheus, and view themselves as guilty. Moreover, since no one wants to take on any more guilt than they already have, no one challenges the power structure. Aegistheus discovers that power has a price: like Jupiter, he becomes his image. He does not know who he is; all he knows is the image he projects for others: a fearful judge. By seizing power he has destroyed himself.
In the Greek myth, Orestes kills Aegistheus out of revenge for his father. In The Flies, however, Sartre emphasizes that Orestes kills him in order to free the people of Argos. Sartre also intentionally places the difference between Agamemnon's murder and Aegistheus's murder in the foreground. Aegistheus killed Agamemnon because he wanted power, not because he felt it was the right thing to do. As a result, he could not take responsibility for his action and passed his guilt onto his subjects instead of accepting it for himself. The point here is to underscore the source of Aegistheus's guilt: Aegistheus is evil not because he killed Agamemnon, but because he knows that human beings are free and he builds up institutions to keep this knowledge from them. Unlike Jupiter, whose essential nature demands that he keep people in slavery, Aegistheus has the choice not to do so, and he chooses to do it anyway. His evil, then, come from the fact that he knowingly deprives others of their freedom.