The importance of the main theme of To Kill a Mockingbird, which is the coexistence of good and evil in society, lies in its universality, that it applies to all places and all times.

In writing her book, Harper Lee wanted to show her readers that though the action of the story takes place in 1930s Alabama, the nature of good and evil and their coexistence is something of great importance to all of us, whoever we are and wherever we live.

Although many readers of To Kill a Mockingbird will only take away the many evils that appear in the book—racism, hypocrisy, injustice, to name but three—there's also a lot of good. As well as unsavory characters like Bob Ewell, there are also genuinely good folks in Maycomb, like Boo Radley, Calpurnia, Miss Maudie, and, of course, Atticus Finch.

Their presence amidst the many evils on display serves as a reminder that evil is never alone; it can only derive its true meaning from its coexistence with what is pure and good.