Jean-Paul Charles Aymard Sartre (1905 - 1980) was a leader in the French philosophy of his time in existentialism, as well as in Marxism. Before we tackle his idea of freedom, and in order to fully understand its impact, we must first have a general view of some important thoughts that circled the cultural society up to the 20th century.
You need to become what you are. Man is not born a man, but he later becomes one. How does he do that? Well, he does it by guiding himself towards the perfect archetype.
This idea was correlated with another notion stating that all individuals have no choice, but to try to copy, in the highest degree possible, that archetype or essence. Of course, man still could lose his essence by moving away from it, and thus, we talk about inhuman acts, but can never break from it entirely.
This philosophical theory has two very important consequences: on one hand, every human being has to aim for the same essence that can never be changed, and on the other hand, man has to find himself in something exterior to his being. Therefore, in a twisted way, the individual (person A, or B, or C) finds himself in the universal (The Human Archetype).
Sartre disagreed with his predecessors and found almost all their ideas - to put it mildly - rubbish. He rejected the idea of the Human Archetype. For him, there can never be a predetermined image of what somebody should be.
Moreover, because there is no ultimate essence as to man is free to invent and reinvent himself, this philosopher tries to save the individual from unifying with the general, and hence ceasing to exist.
Also, because there is no human order, in the same sense as there is an order of objects studied and analyzed by scientists, man can never be the object of observation or scientific study. He is the original product of a free choice. The only loyalty he owes is that reserved to himself.
The only law/rule a man needs to obey is that of the ideal he freely accepted as his own. A man is and can never be anything else than what he himself decides to become. Therefore, to be is to choose because "existence precedes essence."
What the philosopher is trying to explain is the fact that each individual must have his own values. In other words, he is the one who judges if the life of a drunk man is more or less important than the life of the President. The problem posed by the relationship between necessity and freedom is solved unilaterally in the favor of the latter.
However, Sartre didn't deny the fact that there are certain inevitable constraints that act upon the individual: the need to work, and to act in the condition given by a specific environment. Every human being present in this world is limited by a situation.
Despite this fact, the philosopher argued that every single situation is open to freedom, because the individual chooses himself by conferring certain significance to it. The leading role is held by our subjective orientation and not by an external necessity.
Let's consider an example of this philosopher: Imagine that you are a soldier taking part in the war. You might be tempted to think that you have no other option but to fight and that you are a victim of destiny. However, he argued that you can always choose to surrender, desert, or even commit suicide.
By refusing to opt for one of these solutions, you are accepting the war in all its forms, and it is your own war that bears your resemblance. This is because, in every situation, the individual is free from the constraints of any influence. He is also fully responsible for his actions and decisions.
So, you can do whatever you want, but you have to face all the consequences. There is no one to defend you, and you have only yourself to blame. The whole weight of the world is on your shoulders, and no one and nothing can change that ever.
To conclude, Sartre believed that a man can never be guilty of inhuman acts, because there is no such thing. Every move that he makes is profoundly human by the simple fact that he is always choosing himself. In fact, each situation he participates in becomes his situation.