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.