Mayella and as a result, he was wrongfully accused, convicted, and killed. Tom only tried to help and never harmed anyone: Miss Maudie's definition of a mockingbird. Byanalogy, it was also a sin to kill Tom Robinson. Boo Radley also never did anyone any harm. In fact, there are numerous times in the novel when he helps Scout and Jem. Therefore, it would be a sin to harm or kill Boo as well. One might consider Atticus as symbolic of the mockingbird as well. He practices what he preaches: justice and honesty. He helps people in any way he can. And he even refrains from fighting back when Bob Ewell spits on him. Atticus generously reasons that to fight back would contribute to more violence: 

So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that’s something I’ll gladly take. He had to take it out on somebody and I’d rather it be me than that houseful of children out there. You understand? (Chapter 23) 

A mockingbird is an innocent creature that is targeted without deserving it.

When Atticus gives his children guns for Christmas, he tells them not to shoot at mockingbirds.  This becomes a metaphor for Scout.  Symbolically, a mockingbird does no harm.

Miss Maudie explains why Atticus tells his children not to shoot at mockingbirds.

"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." (ch 10)

There are two symbolic mockingbirds in the story: Boo Radley and Tom Robinson.  Boo Radley is actually identified as a mockingbird by Scout at the end of the story.  Scout tells Atticus that she understands why Heck Tate decided to say Bob Ewell fell on his knife, rather than identify Boo Radley as his killer—even though he was saving the children’s lives.

"Well, it'd be sort of like shootin' a mockingbird, wouldn't it?" (ch 30)

Scout describes Boo as a mockingbird because he does no harm, and has been only a joy in the children’s lives.  He is targeted by society for no reason other than that he is different.  Similarly, Tom Robinson is targeted because of his race.

The first reference to mockingbirds in the book occurs when Jem and Scout get air rifles from Uncle Jack for Christmas in chapter 10. Atticus is not too thrilled by this gift but understands that it is a common toy for children. He instructs his children that they should never use their new air rifles to kill a mockingbird because it would be a sin to do so. Scout is struck by her father's instructions because he never calls anything else sinful. Miss Maudie explains just what Atticus means when she tells Scout,

Mockingbirds don't do one thing but make music for us to enjoy … That's why it's a sin to kill a mockingbird.

Throughout the book, two people are overtly and subtly compared to mockingbirds. They are nice to others but are commonly misunderstood and ill-treated. The first is Tom Robinson. He is a kind man who goes out of his way to treat everyone well, even those who wish him harm. Despite this, he is falsely accused of rape, convicted in court, and killed. The other mockingbird is Arthur (Boo) Radley. This reclusive neighbor of the Finch family seems to desire nothing but to be left alone. He does, however, surreptitiously do nice things for the kids, such as mend Jem's torn pants and leave them gum in the tree hole. After Boo kills Bob Ewell to save Jem and Scout, Atticus and Sheriff Tate decide to hide the truth to protect Boo from unwanted attention. With this, Scout makes a direct connection to what her father said about mockingbirds:

Mr. Tate was right … It would be sort of like shooting a mockingbird, wouldn't it?

The title of the novel To Kill a Mockingbird refers to a saying that Atticus Finch passes on to his children, Scout and Jem. In chapter 10, after the children witness him kill a rabid dog with one shot, they discuss the ethics of shooting animals. He advises them that it would be acceptable to shoot a blue jay, but “it’s a sin to kill a mockingbird.” As Atticus otherwise does not speak about sin, Scout is confused. She consults their neighbor Miss Maudie. This kindly older woman confirms Atticus’s advice:

“Your father’s right,” she said. “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.”

The idea that killing any creature, human or otherwise, if that being only brings enjoyment is a key theme of the novel.

Two of the characters to whom this advice applies are Arthur “Boo” Radley and Tom Robinson. Atticus is concerned that his children are harassing Boo because their overactive imaginations consider him some kind of monster. Atticus emphasizes his humanity and encourages the children to have compassion. Boo is ultimately shown to be a lookout and protector of the children. Scout demonstrates her understanding of the mockingbird metaphor (chapter 30). Given Boo’s intense introversion, she offers that publicizing his actions would be like killing a mockingbird.

In regard to Tom Robinson, a “songbird” comparison is used by Braxton Underwood in a newspaper editorial after Tom is killed. This usage indicates that Underwood believed in Tom’s innocence, despite his conviction.

When Atticus states that it is a sin to kill a mockingbird, instructing Jem and Scout never to shoot at them with their new air rifles, Scout wonders why this is so. She asks her neighbor Miss Maudie why killing this bird is a sin. Miss Maudie replies that this is because mockingbirds don't do any harm. They simply sing songs that people enjoy hearing.

The novel is called To Kill a Mockingbird because the mockingbird is a symbol of Tom Robinson. Robinson does no harm to anyone. In fact, he is helpful to Mayella Ewell, who he feels sorry for as a lonely young woman. He is also known in the Black community as an upstanding family man. Nevertheless, he is accused and put on trial for raping Mayella.

Atticus is able to prove the accusation is false and that Robinson is innocent. Nevertheless, Robinson is convicted of the crime because he is Black, and the Jim Crow code in the South says that the word of a white woman is always believed over the word of a Black man. Robinson is later killed. It was as much a sin to have killed this innocent man who did no harm as it is to kill a mockingbird.

When Scout and Jem get air rifles for Christmas, Atticus tells them, “Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird.” Scout does not understand what her father means, and this is first time he has ever spoken of anything as being sinful. She decides to ask Miss Maudie, and Miss Maudie explains, “Mockingbirds don't do one thing but make music for us to enjoy.” They never cause any harm; they just “sing their hearts out for us.” Something that is so beautiful and so generous and so peaceful should not be harmed.

The mockingbird is mentioned one more time, much later in the novel. Scout and Jem have been attacked by Bob Ewell, and Boo Radley comes to their aid, killing Ewell in the process. At first, Atticus thinks that Jem might be responsible for Ewell's death, but when Mr. Tate finally makes Atticus understand what really happened, Atticus agrees to let Mr. Tate's explanation stand. Ewell fell on his knife. Period. Mr. Tate is completely unwilling to put Boo Radley into the limelight. If the town were to realize that he is the one who saved Scout and Jem, they would make a fuss over him, wanting to do all kinds of favors for him, thinking they are doing him a service. The reality would be the opposite. Boo Radley would be overwhelmed with “kindness” and miserable.

After Mr. Tate leaves, Atticus asks Scout if she understands about Ewell falling on his knife. She tells him that she does and that Mr. Tate is right. Then she adds, “Well, it'd be sort of like shootin' a mockingbird, wouldn't it?” In her childhood innocence, Scout understands that Boo Radley is like a mockingbird. He causes no harm; he merely wants to brighten the world in the limited ways in which he knows how. He is, beneath his reclusive exterior, beautiful and generous and peaceful, so he should not be harmed.