They're free; and human life begins on the far side of despair.

This is the sort of cheery pessimism that leads people to complain that existentialism is a philosophy of adolescent angst. In Act III, Orestes tells Jupiter that he will share his despair with the Argives because it will enable them to live their lives. The meaning of "despair" here is not self-explanatory. Following moral laws is comforting: one does not have to take responsibility for one's actions. So long as one's actions follow some law, the responsibility for those actions lies with the law, not the agent. The Argives, bound to Jupiter's moral law, never have to accept responsibility for their actions. Orestes, having recognized his freedom, understands that regardless of what moral standards may govern a society, every human being still has the freedom to follow those laws or to break them. The presence of moral laws is not an excuse for doing something, since the individual can always violate those laws.

The Argives act as if they have no choice but to follow these laws, in the same way that rocks and trees have no choice but to follow the laws of physics. But human beings can also recognize that it is they themselves who give the laws their strength and that they can change the laws by creating new ones. Human beings can realize that nothing forces them to follow moral laws; they are always free to create new laws. This recognition that one's life is not governed by certain laws and that one has the ability and responsibility to create one's own laws is what Sartre means by despair. The comfort of knowing that one is always doing what is right according to certain laws is swept away. Instead, one always feels the anguish of knowing that there is no overriding authority that can determine who is right and who is wrong: moral judgment comes from us. Once we experience despair, i.e., once we recognize that the laws we follow are not definite and fixed, we are free to shape our own lives and create our own values. True human life can only begin once we recognize our freedom.