Atticus does not volunteer to defend Tom Robinson. Instead, he is assigned the case by Judge Taylor, who
"... pointed at me and said, 'You're It.' "
Atticus would have preferred not to have been the judge's choice, and he confided in his brother, Jack, that
"I'd hoped to get through life without a case of this kind..."
But Atticus has another reason for not refusing his assignment: He does it to set an example for his children.
"... do you think I could face my children otherwise?... I hope they trust me enough..."
Atticus recognizes that Tom is innocent, and he is able to see what the jury apparently does not: That Tom could not have committed the crimes of which he was accused because of his crippled left arm. Atticus also understands that the left-handed Bob Ewell must have beaten his own daughter. Atticus has taught his children that the "worst thing" a white man can do is "cheatin' a colored man," and he aims to exonerate Tom from the trumped-up charges made by the Ewells.