Chapters 10-11: Why is it appropriate to kill a mad dog but not a mockingbird? Explain.

One day after Christmas, Jem and Scout go out to look for some small prey to hunt with their air rifles. Jem is alarmed when he looks down the road to see a neighborhood dog walking "erratically, as if his right legs were shorter than his left legs," (Lee 123). Jem and Scout run to tell Calpurnia, and she immediately calls Atticus and has Eula May call the neighbors to warn them that a mad dog is on the loose. Atticus shows up with Mr. Heck Tate, the sheriff, and they wait for the dog to come within shooting distance. Atticus, to the surprise of Jem and Scout, is asked to shoot the dog, and he  kills the mad mongrel with a shot right above the left eye. Atticus had told the children earlier that they should never shoot a mockingbird, but then he goes and shoots a diseased canine. The difference is that mockingbirds "don't do one thing but make music for us to enjoy," (Lee 119). The mad dog could've been a threat to the community, and might have attacked a human. It's okay to defend yourself against something that might attack you, but it is not okay to go out of your way to harm something that has never done anything wrong. I think part of the bigger picture with this situation is that rules change depending on who's playing the game. Everybody has a different background and reasons, so you can't determine right or wrong without knowing the background. If the dog hadn't been mad and Atticus had shot it, he would've been in trouble. If a child shot a bird that was about to peck it's eyes out, that could probably be understood. That's why it's okay for Atticus to kill a dog that was dangerous, but it's not appropriate for children to kill an innocent mockingbird just for fun.

One question I have is how this relates to the rest of the book. Will Tom Robinson be the innocent mockingbird that Mayella Ewell decides to prosecute, or will he be the dog that didn't hurt her, but there was a strong possibility he might've if she hadn't done anything? Is he even one of the two or something completely different?