"The Outsider" explores Albert Camus's interest in the philosophy of Existentialism, which was most famously articulated by Camus's contemporary Jean-Paul Sartre. Briefly, Existentialism is based on the existence of the individual rather than any abstract concept (such as Plato's Realm of the Ideal.) The individual lives in a world which is inherently absurd, since fear, pain, hunger and so on exist alongside our craving for life and happiness. The only way for individuals to live is, not according to a system or code (as most philosophies and religions believe) but authentically. That is, you should admit to your ideas and emotions, and not try to hide or change what you are in response to social conventions.
Meursault, the protagonist of the Outsider, embodies this attitude. Having committed a senseless (absurd) act of murder, he finds that he is condemned not so much for the murder, as for his refusal to show "proper" emotions. Rather than live a lie, he accepts the death sentence, and before his death, having fully accepted himself and his world, feels fully alive for the first time.
Meursault, the protagonist of the Outsider, embodies this attitude. Having committed a senseless (absurd) act of murder, he finds that he is condemned not so much for the murder, as for his refusal to show "proper" emotions. Rather than live a lie, he accepts the death sentence, and before his death, having fully accepted himself and his world, feels fully alive for the first time.