In Chapter Twenty, Atticus addresses the jury at the end of Tom Robinson's trial. In his summation he veers away from the crime that Tom Robinson is supposed to have committed. He instead focuses on Mayella Ewell and what she has done. It is clear that Atticus does not see her as a victim but as a perpetrator and, therefore, depicts her as such.

Atticus does not directly state that Mayella has committed a crime, but through careful and meticulous wording, suggests that, firstly, Mayella has transgressed an unspoken code. This code is found on racist principles which prohibit any intimate contact between whites and blacks. In the society that she lives, it is absolutely taboo for there to be anything but superficial associations between whites and blacks. Atticus says:

My pity does not extend so far as to her putting a man’s life at stake, which she has done in an effort to get rid of her own guilt.

He is clearly suggesting that Mayella's actions are criminal. Atticus refers to her actions with regard to her liaison with Tom Robinson as an "offense" and states that:

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.

Mayella obviously knows what she is doing but feels compelled to persist in accusing Tom Robinson. She fears rejection by her own kind and does not want to be shamed and ridiculed. Atticus suggests that she is driven by 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.

The crime that Atticus is, therefore, alluding to is the fact that Mayella Ewell is implicating an innocent man and putting his life at risk. She does this to save herself from being hounded out by her society. She wants to break away from being held responsible for her so-called offense and the only way she can do so is by getting rid of Tom Robinson, as Atticus states before making his final closing remarks:

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.

It is tragically sad and disparaging that Atticus's most ardent appeal to the jury to apply common-sense, and his request for them to invoke the principle of equality, fall on deaf ears for, in the end, Tom is found guilty and sentenced to death.  

It was in Chapter 20 when Atticus mentioned the "crime"--however, he called it a "code" rather than a crime that was broken.

"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."

He continues on in Chapter 20 explaining that she knew what she was doing.  She didn't plan it from the beginning, but by accusing Tom, she was getting herself out of trouble with her father (and society--although she's already an outcast from society).

"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."