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

The doctor returns and everyone moves to the back porch. Trying to be as friendly as possible, Scout leads Boo to the porch and assists him into a rocking chair placed in a darker corner, where she thinks he will feel most comfortable. As she helps Boo along, she feels the odd sensation of her fantasy about finding him sitting on the porch one day coming true. Meanwhile, the others are discussing who killed Mr. Ewell. Atticus thinks that Jem must have done it since Scout named Jem as her protector in her story. However, the sheriff insists continually that Mr. Ewell fell onto his knife and killed himself, which irritates Atticus, who wants Jem to be treated as fairly as anyone else and not have exceptions made. After much arguing, finally the sheriff yells out that he's not trying to protect Jem (he is trying to protect Boo). The sheriff urges Atticus, this once, to accept the situation even if it's not perfect according to law: Mr. Ewell was responsible for Tom's death, and the sheriff urges Atticus to "let the dead bury the dead." He says that it would be a sin to drag shy Boo Radley out into the limelight, and declares officially that Mr. Ewell fell on his own knife. Atticus, deeply moved by this revelation, asks Scout if she understands. Scout assures him that she does, explaining that having it another way would be like shooting a mockingbird.

The doctor returns and everyone moves to the back porch. Trying to be as friendly as possible, Scout leads Boo to the porch and assists him into a rocking chair placed in a darker corner, where she thinks he will feel most comfortable. As she helps Boo along, she feels the odd sensation of her fantasy about finding him sitting on the porch one day coming true. Meanwhile, the others are discussing who killed Mr. Ewell. Atticus thinks that Jem must have done it since Scout named Jem as her protector in her story. However, the sheriff insists continually that Mr. Ewell fell onto his knife and killed himself, which irritates Atticus, who wants Jem to be treated as fairly as anyone else and not have exceptions made. After much arguing, finally the sheriff yells out that he's not trying to protect Jem (he is trying to protect Boo). The sheriff urges Atticus, this once, to accept the situation even if it's not perfect according to law: Mr. Ewell was responsible for Tom's death, and the sheriff urges Atticus to "let the dead bury the dead." He says that it would be a sin to drag shy Boo Radley out into the limelight, and declares officially that Mr. Ewell fell on his own knife. Atticus, deeply moved by this revelation, asks Scout if she understands. Scout assures him that she does, explaining that having it another way would be like shooting a mockingbird.

http://www.gradesaver.com/to-kill-a-mockingbird/study-guide/section5/

The doctor returns and everyone moves to the back porch. Trying to be as friendly as possible, Scout leads Boo to the porch and assists him into a rocking chair placed in a darker corner, where she thinks he will feel most comfortable. As she helps Boo along, she feels the odd sensation of her fantasy about finding him sitting on the porch one day coming true. Meanwhile, the others are discussing who killed Mr. Ewell. Atticus thinks that Jem must have done it since Scout named Jem as her protector in her story. However, the sheriff insists continually that Mr. Ewell fell onto his knife and killed himself, which irritates Atticus, who wants Jem to be treated as fairly as anyone else and not have exceptions made. After much arguing, finally the sheriff yells out that he's not trying to protect Jem (he is trying to protect Boo). The sheriff urges Atticus, this once, to accept the situation even if it's not perfect according to law: Mr. Ewell was responsible for Tom's death, and the sheriff urges Atticus to "let the dead bury the dead." He says that it would be a sin to drag shy Boo Radley out into the limelight, and declares officially that Mr. Ewell fell on his own knife. Atticus, deeply moved by this revelation, asks Scout if she understands. Scout assures him that she does, explaining that having it another way would be like shooting a mockingbird.