First, it is important to define what a mockingbird is. Fortunately for us, the book does this for us. Here is a conversation that Atticus has with Jem and Scout has with Miss Maudie.
To Kill A Mockingbird
Atticus said to Jem one day, “I’d rather you shot at tin cans in the back yard, but I know you’ll go after birds. Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird."
That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it.
“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.”
If we take this to be the meaning of a mockingbird, then the most obvious mockingbird is Tom Robinson. He only did good things, even for the Ewells. He was a kind and good man. Therefore, to harm him is to harm a mockingbird, which is a sin.
The kids are also mockingbirds. Jem and Scout are innocent and kind. So, when Bob Ewell tried to kill them, he was committing a sin as well.
At the end of the book, Boo Radley is also seen as a mockingbird. He is an innocent man and so to harm him or fail to protect him is to fail to protect a mockingbird. It is amazing that Scout, as a child, saw this dynamic. She saw is better than Atticus! Here is how the book ends:
Atticus sat looking at the floor for a long time. Finally he raised his head. “Scout,” he said, “Mr. Ewell fell on his knife. Can you possibly understand?"
Atticus looked like he needed cheering up. I ran to him and hugged him and kissed him with all my might. “Yes sir, I understand,” I reassured him. “Mr. Tate was right."
Atticus disengaged himself and looked at me. “What do you mean?” “Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it?”