Atticus Finch defends Tom Robinson in To Kill a Mockingbird by Harper Lee. Living in Maycomb, Alabama, in the 1930s means that Atticus will have to defend not only Tom but his beliefs and his family as well. Prejudice in Maycomb was just as strong as it was in 1864 during the Civil War. Many of Maycomb’s citizens believe that black people are sub-human.
The trial of Tom Robinson gives Atticus opportunities to teach the children about prejudice and its impact on people both black and white. Jem, as he matures, begins to understand the ugliness of bigotry as seen inside and outside the courtroom during the trial.
Why did Atticus defend Tom Robinson when he knew that he could not win the case? This unique man explains it to his children:
"If you shouldn't be defendin' him, then why are you doin' it?"
"For a number of reasons," said Atticus. "The main one is, if I didn't I couldn't hold up my head in town, I couldn't represent this county in the legislature, I couldn't even tell you or Jem not to do something again…Scout, simply by the nature of the work, every lawyer gets at least one case in his lifetime that affects him personally. This one's mine, I guess."
Integrity and character are two of the traits that Atticus tries to instill in his children. He must demonstrate these qualities himself. Atticus commits to treating everyone the same. He does not believe in violence which often occurs in the story.
As a lawyer, Atticus promises to defend anyone that needs his help. The judge assigned him to the case, but Atticus would have taken the case anyway.
The case cannot be won at this time in the south. Neither women nor black people serve on the juries. Everywhere a person turns there is bias----against women, blacks, people like the Cunningham and the Ewells.
To definitively answer the question, Atticus defends Tom Robinson because it is the right thing to do. There will no change in prejudice until everyone receives the same treatment in the court system which Atticus states “is the great leveler of all people.” All people deserve a fair trial and a fair judgment by their peers.
Tom Robinson did get the trial in which everything that needed to be said was stated. One small step was taken: the Cunningham man did not want to vote guilty. The jury stayed out longer than any other black person's jury. Atticus knew that even one juror might lead to the next trial where five jurors might vote not guilty.
As Mr. Dolphus Raymond tells Scout in front of the court house: “…you don’t your pa’s not a run-of-the mill man…”