How is the theme of coexistence between good and evil portrayed in To Kill a Mockingbird?
Evil coexists with good in the town of Maycomb, particularly the evil of racism. Despite Atticus's best efforts, Tom Robinson, a black man, is found guilty of a crime he didn't commit simply because of his race. The people in the town are shown to be hypocrites who put upholding racial codes ahead of mercy.
Yet, the book argues, the people of Maycomb aren't wholly evil and part of what Jem and Scout have to learn is that the world isn't split into "good" people and "evil" people. Almost everyone is a mixture of both. It's important to Atticus that the children learn this lesson and learn to be tolerant rather than judgmental.
One example of a person who is a mix of good and evil is Mrs. Dubose. Jem and Scout hate because she is portrayed as an evil, ugly old woman, the neighborhood witch, who is always criticizing and insulting them. Yet when Jem knocks the heads off of her camillias (the state flower of flawed Alabama and a symbol of Mrs. Dubose, who is as racist as her state) with Scout's baton, Atticus makes them read to her. The children learn to admire her for courage in fighting off her morphine addiction: they learn she is a mixture of both good and evil.
Likewise, good and evil coexist in Mayella Ewell. She is evil to lie about Tom making sexual advances to her, but she is also a person we feel sorry for, struggling to survive in poverty with an abusive father.
By offering such a full account of Maycomb and its people, Harper Lee attempts to help us understand how complicated humans can be. Lesser works of literature have what are called "cardboard characters," characters who are all good or all evil. While this may be more comfortable for us than a mix of good and evil in the same person, Lee challenges us to see that the world and the people in it, in reality, consist of this mix.