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Mayella is a victim. Atticus is being as courteous as he can with her but the absence of truth in her confession is glaring. She is under great pressure to lie for her father. When her lies get all mixed up in her head (and Atticus exposes them) , she breaks down.
i think she was trying to make Atticus to stop asking her question since she is afraid that it might happen that he end up talking the truth about the person who raped her(Mr Ewell)