A staggering, rabid dog called Tim Johnson is shot by Atticus.

When the Finches' maid, Calpurnia, sees the Johnson dog (whom Jem describes as having "gone lopsided"), she immediately phones the office of Atticus. Then she tells the operator to contact whoever has a phone on the same street as the Finches and inform them about the rabid dog. After she hangs up, Calpunia runs outside, shouting to whomever will hear her. She knocks on the Radleys' front door to no avail. As she walks back to the house, she sees a black vehicle turn into the Finch driveway. Atticus emerges with Sheriff Tate.

"Not runnin', is he?" asked Mr. Tate.
"Naw, sir, he's in the the twitchin' stage, Mr. Heck."
"Should we go after him, Heck?" asked Atticus.

In a suspenseful moment, Sheriff Tate suggests that they wait and watch the dog. Sheriff Tate and Atticus move nervously as Miss Stephanie regards the action from the glass window of her front door, while Miss Maudie stands beside her.

Soon, the dog reaches the side street in front of the Radley Place. He tries to step forward, then turns slightly. Atticus tells the sheriff that the dog is within range of his rifle; he urges the sheriff, "You better get him before he turns down the side street." But Mr. Tate hands the rifle to Atticus, telling him to shoot because the opportunity to get the dog is only "a one-shot deal." Atticus further urges Mr. Tate to not waste time, but the sheriff points out that if he shoots and misses, the bullet will go into the Radley house. Since he cannot shoot as well as Atticus, the sheriff urges Atticus to shoot. Still Atticus demurs, "I haven't shot a gun in thirty years." Ignoring this statement, Sheriff Tate tosses the rifle to Atticus.

In the meantime, Tim Johnson has changed his mind, and he now pursues his course up the street. Quickly, Atticus brings the gun to his shoulder, takes off his glasses, and shoots the poor dog. "I saw that, One-Shot Finch!" calls Miss Maudie. Atticus merely returns to the car, and he warns Jem not to go near the dog, who is as dangerous dead as when he was alive. Jem replies, "Yes, sir...Atticus--" He starts to ask his father about his skill at shooting a gun, but the words will not come out. He is simply amazed.

As Atticus drives away, Jem realizes, "Atticus is a gentleman, just like me!" Atticus, Jem realizes, keeps things to himself that need not be said. Clearly, Jem's perspective has changed. Maybe his father is not such an old man, after all.

It sounds a bit strange, but Atticus shoots Tim Johnson and kills him.  This sounds strange, but it is not because Tim Johnson is not a human being.  Tim Johnson is a dog.

Tim has gone mad -- presumably he has rabies.  That makes him a danger to everyone in town because if he bites them they will get the disease.  Heck Tate asks Atticus to shoot the dog because Atticus is the best shot around.  With Atticus shooting, there will be less risk of the bullet missing Tim and accidentally hurting a person.