To Kill a Mockingbird explores many aspects of good and evil. Sometimes what represents good and evil is clear. Bob Ewell certainly appears evil, as he is shown to be a self-serving sociopath who does not hesitate to threaten or hurt others as he sees fit. Atticus Finch is a clear representation of good, as he stands up for justice even at great risk to himself, his family, and his reputation.
Harper Lee also explores more subtle concepts of good and evil. The people of Maycomb are by and large decent people. However, within most individuals we meet, there is an element of racism. They are not evil people, but they are capable of evil (racist) thinking and even actions. Through this lens, Harper Lee forces the reader to consider whether most people are inherently prone to being good or evil. Throughout the story, we see that there is nuance and the capability for both inside everyone.
It is possible that even Atticus harbors evil inclinations, or Bob Ewell has some goodness inside of him. However, it is the actions that one takes that define them. In chapter 15, for instance, the lynch mob that gathers outside of Tom Robinson's cell is composed of people from all walks of life. They are bent on evil, though they may see it as an act of justice through their warped racist worldview. However, Scout, recognizing Mr. Cunningham in the crowd, exchanges some nice words with him, and the mob disperses. As readers, we are left to wonder if Mr. Cunningham really is evil.
There are a number of instances like these that force us to examine what makes people do good or evil acts. What I believe Harper Lee intentionally made vague was whether people themselves are good or evil. That is left up to the reader to decide.
The theme of good and evil is developed through Tom Robinson’s trial. Tom and Boo Radley are good, and Bob Ewell is evil.
The most significant element of evil in the book is racism. Racism is presented as an illogical, almost insurmountable evil—like the mad dog Atticus shoots. For most people in Maycomb, hate and prejudice are a part of life. There is nothing immoral about racism. The Robinson trial demonstrates that this is not true. Atticus lectures the jury on racism.
[Some] Negroes lie, some Negroes are immoral, some Negro men are not to be trusted around women- black or white. But this is a truth that applies to the human race and to no particular race of men. (ch 20)
Atticus gets the people of Maycomb to consider racism for what it really is.
Although most people in Maycomb are ordinary people, Bob Ewell is the worst sort of racist. Even so, Atticus tells Jack, the “jury couldn't possibly be expected to take Tom Robinson's word against the Ewells” (ch 9). Bob Ewell is a coarse, difficult man. He does not work, and he drinks his relief checks away. He beats his children and has to hunt out of season just to feed them.
Despite Bob Ewell’s racism, young Mayella is not a racist. She is lonely, and she seems to count Tom Robinson as a friend. Her father can’t stand that. When Atticus defends Tom, Ewell takes it as an affront and tries to kill his kids.
Boo Radley, whom the children think is evil, defends this. This demonstrates how evil is not in what we don’t understand, but in what we accept.