One of the more obvious answers to this question would be the mockingbird. Jem and Scout receive air rifles for Christmas. In chapter 10, Atticus instructs the children to "shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird." Miss Maudie confirms this thinking when she tells Scout that mockingbirds "don't do one thing but sing their hearts out for us."
In chapter 25, in The Maycomb Tribune, Mr. Underwood's editorial compares the death of Tom Robinson to the, "senseless slaughter of songbirds by hunters and children." Tom is a second example of one that does good. His life severely changes for the worse when he performs a kind gesture. According to Scout, "Tom was a dead man the minute Mayella Ewell opened her mouth and screamed."
In chapter 30, Atticus and Heck Tate agree to stick to the story that Bob Ewell fell on his own knife. Concerned about Scout's possible reaction to this, Atticus asks her if she understands. Scout does understand. Boo has done nothing but good for Scout and Jem. He leaves gifts for them in the tree, covers Scout with a blanket on the night of the fire, and saves their lives. To put Boo through all of the public attention he would receive if this news gets out would be, according to Scout, "sort of like shootin' a mockingbird."