Sartre draws two important conclusions from this. First, he argues that there is no given human nature common to everyone because there is no God to give it in the first place. Human beings exist, that much is clear, but it is only after they exist that some essence that can be called "human" may develop. Human beings must develop, define, and decide what their nature will be through an engagement with themselves, their society, and the natural world around them.
Second, Sartre argues that because the nature of every human being is dependent upon that person, this radical freedom is accompanied by an equally radical responsibility. No one can simply say "it was in my nature" as an excuse for some behavior of theirs. Whatever a person is or does is wholly dependent upon their own choices and commitments - there is nothing else to fall back upon. People have no one to blame (or praise) but themselves.