To Kill a Mockingbird is largely remembered of in terms of the trial of Tom Robinson and its racist outcome. For this reason, people often think that the book's theme is simple, a straightforward criticism of racism and evil. But To Kill a Mockingbird is actually more complicated (and interesting). Except in the case of Bob Ewell, the novel avoids simple portrayals and criticisms of "evil." Instead, it shows through Scout and Jem's experiences that Maycomb and its citizens are a complicated mixture of good and bad, full of people with strengths and weaknesses.
There are two characters of almost complete good in To Kill a Mockingbird: Atticus and Boo Radley. But they are good in different ways. Boo maintains his goodness by hiding from the world, while Atticus engages with it. Atticus acknowledges the evil in people and the world and fights against that evil, but he also appreciates what is good in the very same people who through fault or weakness might be supporting an evil cause. Atticus believes that everyone has a basic human dignity, and that he therefore owes each person not only respect, but the effort to try to understand their point of view. Atticus tries to instill this worldview in Scout when he tells her that instead of condemning people for doing things that she thinks are cruel, or unfair, or just plain weird, she should first try "standing in their skin."
Atticus's belief in treating and respecting everyone as an individual is contrasted in To Kill a Mockingbird with a number of other worldviews. These other visions are all quite different from each other—they are religious, racist, classist—but they all share one thing in common: they treat people as groups, demand conformity, and give no respect or credit to individuals. In other words, they are all forms of prejudice, which is a preconceived notion about a person based on the groups to which that person belongs. Over and over again, To Kill a Mockingbird reveals prejudice not just as closed-minded and dangerous, but also as ridiculous.
The most obvious form of prejudice in the novel is racism, which causes otherwise upstanding white citizens of Maycomb to accept the testimony of an obviously corrupt white man over the evidence supporting the testimony from a black man. Yet prejudice is also visible in the racially condescending Mrs. Grace Merriweather; in Aunt Alexandra's and many other character's belief in the importance of social class; in the gender stereotypes that people try to force on Scout; and even in the way the town views Boo Radley as a monster because he acts differently from everyone else.
In the three years covered by To Kill a Mockingbird, Scout and Jem grow up. At the start of the book they are innocents, with an uncomplicated sense of what's good (Atticus, the people of Maycomb) and what's evil (Boo Radley). By the end of the book, the children have lost their innocence and gained a more complex understanding of the world, in which bad and good are present and visible in almost everyone. As the children grow into the adult world, though, they don't just accept what they see. They question what doesn't make sense to them—prejudice, hatred, and violence. So while To Kill a Mockingbird shows three children as they lose their innocence, it also uses their innocence to look freshly at the world of Maycomb and criticize its flaws.
Like every kid growing up, Scout attends school for the first time. But rather than contribute to her education, Scout's school is depicted as rigid to the point of idiocy, with teachers who criticize students who got on early start on reading and hate the Nazis but can't see the racism present in their own town. To Kill a Mockingbird does not so much explore standardized school education as condemn it, showing how it emphasizes rote facts and policies designed to create conformist children rather than promote creative critical thinking, sympathy, and mutual understanding across racial and socioeconomic boundaries.
Many people, including Jem and Scout when they're young, mix up courage with strength. They think that courage is the ability and willingness to use strength to get your way. But Atticus defines courage as "when you know you're licked before you begin but you begin anyway and see it through no matter what." Courage, in To Kill a Mockingbird, is not about winning or losing. It's about thinking long and hard about what's right instead of relying on personal prejudice or gut reaction, and then doing what's right whether you win or lose. To Kill a Mockingbird is filled with examples of courage, from Mrs. Dubose's fight against her morphine addiction, to Atticus's determination to face down the racism of the town, to Mr. Underwood's willingness to face down his own racist feelings and support what he knows, in the end, is right.
Maycomb is a small town, with all of the characteristics implicit in small town life: everyone knows everyone else's business, which can lead to endless and mostly harmless gossip, but more importantly makes the community extremely intimate and close-knit. The first part of To Kill a Mockingbird focuses on this close-knit community, because when they're young Scout and Jem believe that's what Maycomb is.
To an extent, the young Scout and Jem are right: Maycomb is a small, safe, peaceful, intimate community. Yet as Scout and Jem grow up, they come to see another side to their small town. They discover that the town has a fiercely maintained and largely illogical social hierarchy based on wealth, history, and race; ensures its safety through a communal insistence on conformity that subjects anyone who does not conform to dislike and mistrust; and gains its peace by resisting change and ignoring injustice. This is not to say that To Kill a Mockingbird is a condemnation of small town life in the South. Rather, the novel sees the town in much the same terms it sees individuals: as containing wisdom and blindness, good and evil, and for all of that possessing its own special dignity.