What brave thing does Atticus do in Chapter 10, and why are Scout and Jem shocked?

One day a dog named Tim Johnson appears in the neighborhood, down the street from the Finch house. He looks strange appearance and walks slowly, with a twitch. The children tell Calpurnia, who takes one look at the dog and immediately calls Atticus to tell him that there's a rabid dog in the neighborhood. Next Calpurnia gets the town operator to call everyone in the neighborhood to warn them. She even runs over to the Radley house and yells a warning to them. Atticus and the sheriff, Heck Tate, drive up, and the sheriff gives Atticus the gun. The dog is so close to the Radley house that a stray bullet might go into the building. Atticus reluctantly takes aim and shoots the dog. The dog crumples into a heap. Jem is dumbstruck with the accuracy of his father's shot. Miss Maudie tells the children that their father used to be known as "One-Shot Finch," the best dead-shot in the county. She says he doesn't shoot unless he has to, because he feels that when he holds a gun, God has given him an unfair advantage over living beings.