Look closely at the reasons atticus gives scout for defending tom robinson. also look at his statement, "every lawyer gets at least one case in his life time that affects him personally.. this ones mine i guess"

Atticus is a man who has a strong sense of justice and self-respect, so he believes that every man should receive appropriate representation in a court of law.  He knows he cannot win the case based on the biases of the town; however, he knows that this case is the one that "affects him personally."  We also feel his desire to show his children the right way to act and the right way to treat other human beings.