Where is this quote found in To Kill a Mockingbird? (Page)
"'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. . . . What did she do? She tempted a Negro.'"
I don't know what edition you are using, but Atticus Finch's closing statement in the trial can be found in Chapter 20 of To Kill a Mockingbird.
The speech is very dramatic in the insistence that Tom Robinson is innocent, but at the same time, is sympathetic to Mayella Ewell's situation and the southern codes of values that dictate how one should act. Tempting a negro was well beyond expectations even for a poor white girl like Mayella. The code of conduct at the time was that the mixing of races was prohibited, and anyone who broke the code would be an outcast. Atticus understands this and tries to break this unspoken social law by pleading to the logic of the jury and imposing the guilt not only on Mayella Ewell but on society as well. Atticus is a man of deep character, and in his final closing argument, he attempts to hold society responsible for Tom's unfair trial.