Innocence isn't something that you earn; it's not something you can get back once you lose it. Innocence is what you're born with. And once you are born you become vulnerable to the evil that can potentially take away your innocence. The infiltration of evil on innocence can be so surreptitious and pervasive. In To Kill A Mockingbird we experience horror in anticipation of this process. It's so inescapable that almost every character we come to know has been influenced by evil. It's the challenge and the hope as we read that our favorite characters will come out unscathed. Unfortunately, that's not how the story unfolds. "Mockingbirds don't do one thing but make music for us to enjoy...They don't do one thing but sing their hearts out for us. That's why it's a sin to kill a mockingbird." (10.90) In this novel, mockingbirds represent innocence. Therefore, killing this bird is a symbol of eradicating innocence. Throughout the book, characters are described as being mockingbirds because they are innocent people being impaired by exposure to evil.
A reader would naturally wish that Boo Radley remain pure of heart. He spends his life in isolation from Maycomb and is thus isolated from burdens that come from society's prejudices. Jem says, "I think I'm beginning to understand why Boo Radley's stayed shut in the house all this time... It's because he wants to stay inside." (23.227) He is one of the novel's mockingbirds because this isolation protects him from evil until Bob Ewell exposes him to it by failing to contain his evil nature as he attempts to stab Jem and Scout in from of his house. At this point, Boo's good nature propels him to come to their rescue. His innocence has now been lost and cannot be restored.