At the beginning of Act III scene 1 in this masterful play, Martha enters alone, after a failed sexual encounter with Nick because he was unable to follow through with the act. So much of this play concerns appearances and deception, and what is unique about this monologue is that, without any audience, Martha appears to be genuine and to actually be real with herself, which arguably is the first time this has occurred in the play. She is actually quite nice about George, and in particular what is highly significant is the following quote:

George who is good to me, and whom I revile... who has made the hideous, the hurting, the insulting mistake of loving me and must be punished for it.

This reveals the true source of Martha's problem with George. Martha hates and despises her husband because he has made the "hideous, the hurting, the insulting mistake of loving me." Her apparent hatred of George and her desire to wound him stem ultimately from her own inability to love herself and accept herself. This is the reason behind her bizarre behaviour, and what is key to realise is that, because this is a monologue, Martha has nobody to act in front of, and the audience sees what is really going on inside of her.