Why des Scout say " Well itd be sort like shootin a mockingbird, wouldnt it? towards the end of chapter 30

Here, yet again, Lee reveals Scout's phenomenal understanding of life. Scout is quite young and her father is not certain if she understands all that was said. However, she surprises him and makes him incredibly proud by comparing Mr. Arthur Radley (Boo) to a mockingbird. Just like a mockingbird, Boo has never harmed a soul, and it would be a sin to bring him to trial for the death of Mr. Bob Ewell, who he killed to protect Scout and Jem.

Here, yet again, Lee reveals Scout's phenomenal understanding of life. Scout is quite young and her father is not certain if she understands all that was said. However, she surprises him and makes him incredibly proud by comparing Mr. Arthur Radley (Boo) to a mockingbird. Just like a mockingbird, Boo has never harmed a soul, and it would be a sin to bring him to trial for the death of Mr. Bob Ewell, who he killed to protect Scout and Jem.

http://www.gradesaver.com/to-kill-a-mockingbird/study-guide/section6/