How does Estelle's obsession with approval from others corrupt her moral character?

Estelle Rigault is the classic example of someone whose whole identity is defined by the look of the Other. In true Sartrean fashion, Estelle has been turned into an object by the look, objectified by how other people regard her. As such, she is not really a person in her own right; she is not able to make a firm commitment to living life the way she wants to in the midst of an absurd, meaningless universe. Hell is other people, as the play's most famous line has it, and other people determine the contours of Estelle's whole existence. Her life is thus mired in bad faith and inauthenticity.
Because Estelle's life doesn't really belong to her, she desperately seeks the approval of others. As she has no true identity of her own, she is forced to seek validation from elsewhere, from the approving gaze of the people around her. In Sartrean terms, she chooses to exist as being-for-others rather than being-for-itself. This means that Estelle has willingly become an object, like all the countless other objects in this meaningless universe. In other words, she is no longer a human subject; she has given up her subjectivity to become a mere object of others' approval. And as morality as such can only really attach to the human subject, Estelle has rendered herself incapable of moral action. By choosing to exist purely and solely for others, by forfeiting her subjectivity, she has corrupted her whole being entirely.