On page 90 of the book, Atticus explains to the children that it is a sin 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 dont' do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird." (pg 90)
Boo Radley had a kind of wild youth, but he has done nothing but been protective of the children. He covered them with blankets when they were outside, and Miss Maddie's house burnt down. He left them gifts in the tree hole, and he sewed up Jem's pants when Jem split them while crawling under the fence. Most importantly, he saved the children from Bob Ewell when Bob Ewell attacked them on their way home from the festival. Even the sheriff says to Atticus,
"Mr. Finch, taking the one man who's done you and this town a great service an' draggin him with his shy ways into the limelight --- to me that's a sin.... If it was any other man it'd be different. But not this man, Mr. Finch." (pg 276)
Atticus realizes what he is saying. It would be like shooting a mockingbird. Even Scout knows that is true. When Atticus finishes this conversation with the sheriff all he can do is thank Boo for saving his children.
Tom Robinson is an equally good man. Mayella would ask Tom to do minor chores around their house in the dump. She was actually flirting with Tom, but he was unaware of it at the time. Atticus asked him in court if he ever was paid for doing these chores.
"No,suh, not after she offered me a nickel the first time. I was glad to do it. Mr. Ewell didn't seem to help her none, and neither did the chillun, and I knowed she didn't have no nickels to spare." (pg 191)
During the trial, Scout notices something about Tom Robinson.
"He seems to be a respectable Negro, and a respectable Negro would never go up into somebody's yard of his own volition." (pg 192)
He was just trying to be a good man and help someone out who needed help. Unfortunately, in order to escape the repercussions of Mayella coming on to a Negro, they charged that Negro with rape. In this case, they did shoot a mockingbird. Tom Robinson was given the death penalty, and he was killed trying to escape the jail yard while he was waiting for his appeal.
Thank You so much!