This is the central chapter of the novel, relating the events which were only hinted at in the first chapter when Lena Grove arrived in town and saw the column of smoke. Earlier, we knew that Christmas had killed Joanna Burden and the intervening chapters have given the motivations and background to the crime.
The first part of the chapter deals with the complete corruption of Joanna Burden. Her and Joe's relationship went through three distinct phases. The first was the seduction which we heard about in the last chapter, then came the wild "throes of nymphomania," and finally, the third phase was Joanna's attempt to change Joe.
During the second phase, Joanna, in the excitement of her sexual relationship with Joe would often cry "Negro! Negro!" emphasizing that she particularly enjoys being corrupted by someone with Negro blood. Thus, in spite of her heritage, which should have conditioned her to accept the Negro as equal, this cry suggests that again Joe is not being accepted as a person of equality. This in itself modifies his relationship with Joanna.
The crucial change comes during the third phase. The reader should remember that Joe always thought of women as being destructive to his sense of order. The dietitian, Bobbie Allen, and unknown prostitutes have forced him to distrust the influence of women who seem to violate his sense of an ordered life. For about two years, Joanna and Joe's relationship conformed to an ordered (though unorthodox) pattern, but when Joanna broke this pattern with her demands that Christmas take over her finances, go to a Negro school, and finally that he pray with her in order to be saved, he again reacted violently to this violation of his concept of an ordered existence. Prayer is particularly offensive to Joe because of his earlier childhood experience with Mr. McEachern when the elder man beat him unmercifully because of his refusal to recite the catechism.
Joe also views women as being capable of destroying his own individuality. He thinks in this chapter that it would be easy to give in to Joanna and live a life of security and ease. But then he thinks that if he did give in, he would be denying everything that he has stood for during his life. Consequently, when Joanna tries to force him to change, he must destroy her or else his own sense of security and isolation is violated, and he loses his own individuality. On the simple plot level, Joe kills Joanna in self-defense because she did attempt to kill Christmas and would have succeeded if the gun had not failed to fire. Thus, in one sense, Joe kills out of self-protection.
He could have run, but again, he has spent his life running and now he feels that he must take his stand and assert his own values even if it means killing the person who is trying to violate his order and peaceful existence.