What is one of the main themes in Harper Lee's To Kill a Mockingbird?

Throughout her novel, Harper Lee consistently juxtaposes good and evil. She utilises Scout and Jem's childhood innocence to show how the concept of evil develops. Initially, Scout and Jem have never truly experienced evil, and therefore believe people are innately good. They believe they understand evil through Boo Radley, as he acts as this looming, mysterious, dark person. Scout and Jem treat Boo Radley as more of a children's ghost story than an actual person. As the story progresses and the children gain more experience, they begin to realize evil is a much more complex concept. They begin to experience racism, deceit, and hatred. At the trial, Jem witnesses evil in the form of racism and deceit as he sees a man willing to condemn another man to death simply because he is black. Scout has her understanding of evil altered when Robert Ewell attacks her to get to her father. That a grown man and a father would be willing to attack a child goes against everything she believes in adults. When she is saved by Boo Radley, this shatters her childlike understanding of evil, replacing her innoncence with a more mature view of the world.