The Question and Answer section for The Souls of Black Folk is a great resource to ask questions, find answers, and discuss the novel.
Black John kills White John because he raped his sister. At the end of his story, we come to learn that Black John is awaiting the lynch noose for the crime he committed against a white man.
Your question is very broad and involved. I suggest you read through Gradesaver's short-summary of the novel in order to understand the main idea. You might also want to read through Gradesaver's detailed summaries and analysis for each chapter.
Can you outline the point for me?