Atticus feels that Tom Robinson deserves the best defense possible.
Although Atticus was assigned to defend Tom Robinson, he is planning to give him a good defense. He understands that Tom is not likely to win, no matter how well he defends him. He knows that the color barrier is just too strong. Yet Atticus believes the only moral thing to do is to defend Tom to the best of his ability.
"Before I'm through, I intend to jar the jury a bit- I think we'll have a reasonable chance on appeal, though..”. (ch 9)
Atticus explains to Scout that just because he knows he is not going to win is not a reason not to try. He tells her he could not hold his head up in town, go to church, or tell his children to mind him if he did not at least try to defend Tom. Everyone deserves a fair trial.
Atticus believes strongly in fair trials. He tells the jury that the court of law is the great equalizer. He also knows that Tom is innocent. He believes he is innocent, and he also believes that he will be convicted. He is willing to do whatever he can, even if he loses.