I think you are talking about the episode with the rabid dog. In the story, a rabid dog, considered the town dog and named Tom Johnson, limps into town showing signs of rabies. Rabies makes an animal vicious and dangerous. It affects the brain and nervous system to the extent that the animal is thought to be insane or “mad”. The dog comes into town and stands at the intersection of two streets. One street leads into downtown Maycomb, the other down the Finch’s street. Calpurnia sees the dog and quickly calls Atticus. When Heck Tate and Atticus arrive, Heck is hesitant to shoot the dog afraid he will miss. He asks Atticus to shoot the dog instead. Atticus aims the gun and kills the dog. It is then that Miss Maudie tells Jem and Scout (who think their dad is pretty boring) that Atticus use to be the best shot in Maycomb County and had the nickname “Ol’ One Shot.” Scout and Jem are shocked to learn this about their father, and they show a new-found admiration for him. It is amazing to Scout and Jem (and to us, the readers) that the seemingly mild-mannered Atticus would be so adept at shooting a gun. It seems out of character.
The dog, Tom Johnson, is used as a symbol of racism. Harper Lee is comparing the disease the dog has to the disease of racism spreading through Maycomb. Atticus is metaphorically “killing” racism in the town of Maycomb by defending Tom Robinson in the upcoming trial.