Tim Johnson is the rabid dog who strolls down the main street of Maycomb in one of To Kill a Mockingbird's most memorable moments (in Chapter 10). Calpurnia alerts the neighbors and Eula May, the town's telephone operator, and soon both Atticus and Sheriff Heck Tate arrive. Since both Atticus and Tate show up in a "black Ford"--Atticus drives a Chevy--we can assume that the sheriff deliberately picked up Atticus on his way there. Tate is aware of the secret that Atticus has withheld from his children--that he was the best marksman in the county as a youth--and the sheriff probably decided to have a better shot handy, if necessary. Atticus probably does not recognize Tate's motives, since he does not want to take the shot. But Tate insists, and he
... handed the rifle to Atticus. Jem and I nearly fainted.
Tate, who "can't shoot that well and you know it!", merely recognizes that Atticus, who
"... if he shot fifteen times and hit fourteen doves he'd complain about wasting ammunition..."
was the better choice to kill the dog. Had Atticus not been present, Sheriff Tate would no doubt have killed Tim Johnson himself.