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.