In chapter 10, Atticus tells his children that "it's a sin to kill a mockingbird." What reason does he give for saying this?
In chapter 10, when Jem and Scout get their air rifles, Atticus is very honest about his feelings on what they can and can not shoot at.
"I'd rather you shoot at tin cans in the back yard, but I know you'll go after birds. Shoot all the blue jays you want, if you can hit'em, but remember it's a sin to kill a mockingbird." That was the only time I ever heard Atticus say it was a sin to do something, and I asked Miss Maudie about it. "Your father's right," she said. "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, the only thing they do is sing their heats out for us. That's why it's a sin to kill a mockingbird."
Atticus just tells them it is a sin to kill a mockingbird, and Scout gets her answer as to why, from Miss Maudie. A mockingbird is just there for us to listen to their sweet music, and that is all. Later on in the story, Boo Radley is compared to a mockingbird. Boo saves Jem and Scout and Atticus knows that if the people of the town found out about it, they would turn Boo into the town hero, and that is the last thing Boo would want. Scout realizes this, as well, at the end of the book. She tells Atticus that if they told what Boo did, it would be like killing a mockingbird.
Atticus says that it's a sin to kill a mockingbird because all they do is sing their hearts out. Mockingbirds don't hurt people, and they are innocent birds. The deeper meaning he is trying to convey is that it's unfair to hurt innocent people. Two particular "mockingbirds" in this book are Tom Robinson and Boo Radley. Neither of them hurt anyone, yet Tom has been accused of rape and Boo Radley has been ruined by his father. By calling the act of killing a mockingbird a sin, Atticus is portraying the need to protect the innocent and the vulnerable. For example, at the end of the book, Heck Tate intends to say that Bob Ewell fell on his own knife to avoid dragging Boo in. Tate realizes Boo doesn't want the attention, and respects this by leaving him alone. Boo is isolated from the town, so the last thing he would want would be to get dragged into something he isn't ready for. Killing a mockingbird is pointless because they don't harm anyone, and the same applies in this example.