Lee's main theme is that it is wrong to kill or harm an innocent being who does nobody else any harm. In the novel, such innocent people are called "mockingbirds." The two main mockingbirds in the novel are Tom Robinson and Boo Radley.
Both Robinson and Radley are good people who are the victims of cruelty.
In Robinson's case, living in a racist society means that, because he is a black man, he will be convicted of the rape of a white woman even though he is obviously innocent of the crime. As Scout comes to understand:
Atticus had used every tool available to free men to save Tom Robinson, but in the secret courts of men’s hearts Atticus had no case. Tom was a dead man the minute Mayella Ewell opened her mouth and screamed.
Boo, as Miss Maudie explains, is the victim of a cruel father who became a religious fanatic and eventually crushed his son's spirit.
Both men show they are kind. Robinson is trying to be helpful to Mayella because he feels sorry for her. Boo reaches out in a variety of ways to Scout and Jem, for example by putting a blanket around Scout's shoulders on the cold night of Miss Maudie's fire. Nevertheless, both are treated with prejudice. Most of the whites in the town can't see beyond Robinson's black skin, while Scout can't see past her conception of Boo as a bogeyman.
Quotes supporting the theme that it is wrong to kill or hurt an innocent creature include when Miss Maudie explains to Scout why Atticus says it is wrong to kill a mockingbird:
Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird.
Mr. Underwood, the newspaper editor, makes the same point when he condemns the murder of Tom Robinson:
He likened Tom’s death to the senseless slaughter of songbirds by hunters and children.