Sartre says, "when we say that a man is responsible for himself, we do not only mean that he is responsible for his own individuality,Sartre says,...

Existentialism does not occur in a vacuum.  Granted, it is not a church or a prescriptive ideology, but it is born by a community into a community.  Paul Tillich says it is the "natural ally of religion."  It is a revolt against traditional philosophy, but it still a public discourse.  Sartre's statement is his hope that existentialism will be carried into the world, not isolated from it.

Existentialism helps an individual determine himself from within instead of being determined by others or outside forces.  But, according to Sartre, this self-determintation and newfound freedom may go to far, at the cost of public responsibility.  To guard against this, Sartre balances his recipe: one part individualism and one part social responsibility.

The great existential fear is that an individual will either deny or overexert his freedom. Having allegiances severed from outside forces (school, family, and God), Sartre says man is "condemned to be free." But, this free will cannot be unadulterated, and it cannot occur in a vacuum.  An existentialist is bound to make responsible choices, which obviously involve others.

In Sartre's play The Flies, the existential hero Orestes must battle God, his family, his tutor, and the townspeople in deciding whether or not to kill his king and mother, Aegisthus and Clytemnestra, the two who murdered his father, Agamemnon.

An easy decision would be to leave the situation alone: a son must not kill his mother; let God punish the murderers.  Or, since he was away when the murders took place and since the murder was not against him personally, Orestes might not have felt responsible to avenge his father's death.

Ultimately, Orestes decides to take revenge, not out of passionate hate or duty as an avenging son, but because of social responsibility: it will free the city from guilt and shame.  Ironically, the townspeople may hate him for liberating them, but that is the price Orestes must pay.  In this way, he is a kind of Christ-figure taking on the sins (Flies) of others.