Towards the beginning of the book, Scout mentions their play guns that they were playing with, and Atticus tells them to never shoot a mockingibird with them. He states that it is a sin to shoot one. Why shoot something that is so innocent, and just there singing its pretty song? It would be an atrocity. At the end, as Tom Robinson is shot trying to escape, the comparison is made--Tom is the innocent mockingbird, just trying to live his life in peace, and they shoot him, and it's a sin. It also symbolizes the jury's decision to find an innocent man guilty of a crime that he didn't commit; they essentially ended Tom's life that day. Another quote for you is in chapter 25, in Mr. Underwood's editorial. In it, “He likened Tom’s death to the senseless slaughter of songbirds by hunters and children."
Tom is an innocent man; he is a good man. He has a wife, kids, and is a hard-working guy just trying to go about his business. He is unfairly targeted just because he is black (just as beautiful birds are unfairly targeted just for their feathers), and taken down as a result. Bob and Mayella target him, accuse him, and the jury and prison guards all buy into that idea that taking Tom down is easy, and not a big deal. Tom is the mockingbird that it is a sin to hurt, but unfortunately, not everyone abides by Atticus' rules of integrity.
I hope that helped; good luck.
Well, one off the bat; "To Kill A Mockingbird is a sin, because a Mockingbird never hurt anybody". "The One Thing That doesn't abide by majority rule is a person's conscience."
“To my way of thinking, Mr. Finch taking the one man who’s done you and this town a great service an draggin him with his shy ways into the limelight to me. That’s a sin it’s a sin and I’m not about t0 my head.”
Need more? Just ask. If you can explain them then your smarted then my English teacher from last year. HA! LOL! :)