In chapters 10 and 11 of Harper Lee's To Kill a Mockingbird, evil is symbolically referred to in order to develop her theme of evil.
In Chapter 10, evil is first symbolized by the children's Christmas gift of air-rifles. Atticus refuses to teach them to shoot, leaving it up to their Uncle Jack instead. The children assume it is because Atticus does not know how to shoot, but later we learn it is because Atticus strongly objects to killing living things, thinking it evil. Atticus's objection to killing living things is first demonstrated in the famous passage that became the basis for the book's title:
I'd rather you shot at tin cans in the back yard, but I know you'll go after birds. Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird. (Ch. 10)
In saying he knows the children will shoot birds, Atticus is implying he understands there are two sides to human nature--evil and good--and that guns have a tendency to bring out the evil nature in humans, as displayed in his innocent children's desire to shoot and kill birds. In addition, Atticus's words imply that it is also human nature to kill or harm those things that are most innocent such as mockingbirds.
Later in the chapter, evil is symbolized through the appearance of the rabid dog named Tim Johnson, who poses a threat to the whole neighborhood. It's at this moment in the story that Atticus astounds his children by demonstrating his sharpshooting skills in order to rescue the neighborhood. Miss Maudie speaks of the evilness of killing when she explains that the reason why Atticus gave up shooting and kept his skills a secret is because he saw that God gave him an "unfair advantage over most living things" through giving him his sharpshooting skills (Ch. 10).
In Chapter 11, evil is represented by Mrs. Dubose's actions. Each time Jem and Scout pass her house, she hurls insults at them and even stoops so low as to say, "Your father's no better than the niggers and trash he works for!" (Ch. 11). Yet, author Lee even uses Mrs. Dubose to show there are two sides to human nature, not just one. She portrays Mrs. Dubose's good side by having Atticus explain upon her death that she was a morphine addict due to her illness yet was devoted to ridding herself of her addiction before she died, despite how much pain she was in. In Atticus's mind, her devotion to her cause made her the "bravest person [he] ever knew" (Ch. 11).