Throughout Blanche's stay at his house, he feels that she has drunk his liquor, eaten his food, used his house, but still has belittled him and has opposed him. She has never conceded to him his right to be the "king" in his own house. Thus, he must sit idly by and see his marriage and home destroyed, and himself belittled, or else he must strike back. His attack is slow and calculated. He begins to compile information about Blanche's past life. He must present her past life to his wife so that she can determine who is the superior person. When he has his information accumulated, he is convinced that however common he is, his life and his past are far superior to Blanche's. Now that he feels his superiority again, he begins to act. He feels that having proved how degenerate Blanche actually is, he is now justified in punishing her directly for all the indirect insults he has had to suffer from her. Thus he buys her the bus ticket back to Laurel and reveals her past to Mitch.
Consequently, when we approach the rape scene, we must understand that Blanche has made Stanley endure quite a bit. She has never been sympathetic toward him. She has ridiculed him. Earlier she had even flirted with him but she has never been his. Thus, when Stanley finds out that she has slept so indiscriminately with so many people, he cannot understand why she should object to one more. Thus, he rapes her partly out of revenge, partly because one more man shouldn't make any difference, and finally, so that she will be his in the only way he fully understands.
Stanley, then, is the hard, brutal man who does not understand the refinements of life. He is controlled by natural instincts untouched by the advances of civilization. Thus, when something threatens him, he must strike back in order to preserve his own threatened existence. If someone gets destroyed, that is the price that must be paid. It is the survival of the fittest, and Stanley is the strongest.
