In Chapter 20 of To Kill a Mockingbird, Atticus is giving his closing statement. Atticus has proven that there is no way Tom Robinson is guilty of the crime Mayella has accused him of, yet he also knows that in a town like Maycomb, the chances of Tom being found innocent are very slim. Atticus goes on to say that Mayella is the one who committed a crime. Mayella is a white woman who tried to tempt a black man. In this society, people just didn't do things like this, and Atticus says that Mayella knows this and tried to cover up her guilt.
"I say guilt, gentlemen, because it was guilt that motivated her. She has committed no crime, she has merely broken a rigid and time-honored code of our society, a code so severe that whoever breaks it is hounded from our midst as unfit to live with. She is the victim of cruel poverty and ignorance, but I can not pity her: she is white. She knew full well the enormity of her offense, but because her desires were stronger than the code she was breaking, she persisted in breaking it. She persisted, and her subsequent reaction is something that all of us have known at one time or another. She did something every child has done- she tried to put the evidence of her offense away from her. But in this case she was no child hiding stolen contraband: she struck out at her victim- of necessity she must put him away from her-he must be removed from her presence, from this world. She must destroy the evidence of her offense."
Atticus is trying to show the jury that Mayella was the one who started all of this, and when she got caught, she tried to make Tom look guilty, and because she was white and Tom was black, it worked. Tom will be convicted for what Mayella started.
"She was white, and she tempted a Negro. She did something that in our society is unspeakable: she kissed a black man. Not an uncle, but a strong young Negro man. No code mattered to her before she broke it, but it came crashing down on her afterwards."