What quote supports the idea that Boo Radley and Tom Robinson are mockingbirds?

At the end of the novel, in chapter 30, Scout learns that the person responsible for saving both her and Jem from the clutches of an angry Bob Ewell is none other than their mysterious neighbor, Boo Radley. She now comes to view him not as a scary phantom monster but as a kind and courageous protector and friend. She also learns from Atticus that to expose the shy and reclusive Boo as a hero to the general public would mean bringing him unwanted acclaim and attention and this would not be the right way to treat him after all that he did for them. Scout also remembers what she had been told before about never killing a mockingbird, since all that they ever do is bring goodness and light into our lives with their innocent yet beautiful singing. She now sees that Boo Radley is like the mockingbird because he had been the one to always leave little gifts for the children and although they taunted him, in the end he protected them from danger. Therefore, she now understands that he should not be harmed in any way. That is why she says,

"Well, it'd be sort of like shootin' a mockingbird, wouldn't it?"

Although this quote refers to Boo Radley at this point in the novel, it can also be said of Tom Robinson because he, too, was a character who was always good and kind, especially to Mayella Ewell when she asked for his help. He didn't deserve to be killed.

The metaphor of a mockingbird is used to represent both Tom Robinson and Boo Radley and this quote from Scout shows that she has come to understand why we must strive to protect the rights of those who are worthy and yet unable to protect themselves from the bad elements in our society. In saying those words, Scout has come of age and although she has lost her innocence, she has gained empathy and compassion for others.