When Jem and Scout received air guns, Atticus tells Jem,
"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." (pg 90)
Scout goes to Miss Maudie to ask her why Atticus would say that was a sin. She tells Scout,
"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 sings their hearts out for us. That's why it's a sin to kill a mockingbird." (pg 90)
When the children are attacked, Sheriff Tate and Atticus have a discussion about who was responsible. It becomes evident that Boo Radley killed Bob Ewell while defending the children. Sheriff Tate wants to keep that news quiet because,
".....taking the one man who's done you andthis twon a great service an draggin' him with his shy ways into the limelight ---- to me, that's a sin." (pg 276)
Arthur Radley is seldom seen. He has never done anything but give the children gifts in the tree, mend Jem's pants when they got caught on the fence, and save the children's lives when Bob Ewell attacked them. He has never done anything but good for the children.
When Atticus thought about it, he told Scout that Bob Ewell did fall on his knife. Scout said she understood. When Atticus asks her what she understands, she replies,
"Well, it'd be sort of like shootin' a mockingbird, wouldn't it? (pg 276)
Arthur (Boo) Radley is being compared to the mockingbird.
I think that this is a point of some contention among literary critics. Think back to the original quote around the time that Jem and Scout receive rifles for Christmas (I believe this is said by Miss Maudie) about how mockingbirds are to be left alone. Mockingbirds don't do anything but make music for the pleasure of others.
Think about which characters in the novel seem to exist (at least within the context of the book) only to help other people, but end up hurt or maybe even dead. Remember too that Chapter 30 is in Part II, which focuses on the trial, but you may find another character who comes under attack earlier in the book.