Throughout the novel Atticus imparts many little lessons to Jem and Scout. Most of them deal with how to treat people and can be summarized fairly simply: treat everyone with respect and dignity.
One example of how Atticus lived that mantra is his acceptance of the Tom Robinson case. Most lawyers around the Maycomb area would not have taken the case. And any court-appointed attorney likely would not have put as much effort into defending Tom as Atticus did. That's precisely why the judge asked Atticus to do it in the first place. Despite the initial evidence, Atticus never treated Tom as a criminal. Many of the community expressed racially motivated hatred against Tom too, but Atticus never did that. In fact, Atticus never treats any race differently, and his treatment of Calpurnia is a good example. She is treated like a member of the family, not a servant.
Another good example of Atticus treating everyone with dignity and respect is how he handles his encounter with Bob Ewell after the case. Bob finds Atticus in the street one day and hurls insults at Atticus and spits in his face. Atticus simply wipes his face off and walks away. He isn't even angry about it afterward. If anything, he acts a bit sympathetic to Bob Ewell's situation. Scout and Jem can hardly believe it, but it is a lesson that sticks with them.
“He meant it when he said it,” said Atticus. “Jem, see if you can stand in Bob Ewell’s shoes a minute. I destroyed his last shred of credibility at that trial, if he had any to begin with. The man had to have some kind of comeback, his kind always does. So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that’s something I’ll gladly take. He had to take it out on somebody and I’d rather it be me than that houseful of children out there. You understand?”