The phrase "it's a sin to kill a mockingbird" refers to intentionally and pointlessly destroying something that does no harm. The mockingbird is a songbird, not a pest, and it isn't a game bird. Killing a mockingbird serves no purpose, and therefore is an act of unnecessary cruelty. When the jury convicts Tom Robinson of rape despite the absence of physical evidence and despite Atticus’s compelling defense, the jury is guilty of the same unnecessary cruelty. The jury specifically, and the town of Maycomb generally, destroy a good person who has never done harm simply because of the color of his skin. Though Tom is the symbolic mockingbird at the heart of the novel, he is not the only character who fits that description. Heck Tate also specifically describes Boo Radley as a mockingbird, in that he is a harmless person who is the victim of pointless cruelty. Unlike Tom Robinson, Boo Radley is not destroyed, though he does suffer greatly.