to protect his children and the other community members. Atticus shooting Tim Johnson parallels his defense of Tom Robinson. Both events place Atticus in a precarious situation and involve him protecting individuals from something dangerous and contagious. Harper Lee masterfully parallels these events when Scout experiences a flashback before the verdict is read.

I saw something only a lawyer’s child could be expected to see, could be expected to watch for, and it was like watching Atticus walk into the street, raise a rifle to his shoulder and pull the trigger, but watching all the time knowing that the gun was empty.

Despite Atticus's best efforts, he cannot protect his client from Maycomb's racism, and Tom Robinson becomes a victim of racial injustice.

The dog itself symbolizes racism.  Atticus's willingness to shoot the dog, parallels his willingness to take on Tom Robinson's case.  The dog is described as  being just as dangerous dead as alive.  So, too, is the racism in the town.  While Atticus may attack that racism in court, no matter what the outcome of the trial, the racism is still rampant, still dangerous whether dead (an acquittal) or alive (a conviction).

Harper Lee was an excellent storyteller.  She not only had a gift for characterisation, but she also was good at setting the tone in a story and creating suspense.  She inserted several Gothic elements in the story like the snow which was unusual for that part of the country, and Tim Johnson the mad dog.  This was part of the theme of good vs evil.  Jem and Scout were often embarrassed because Aticus was so much older than many of the other dads, especially Jem.  The insertion of the mad dog who Aticus says, "is just as dangerous dead as alive," was a brilliant way to show good, brave (Aticus) overcome evil (the dog).  The secondary affect was that and Scout see their father in a new light and amazed to find out that his father was one of the "best shots" in the county.

According to the story, one Saturday Jem and Scout are out exploring and hunting for squirrels with their air rifles. Scout notices that Jem is looking at something and when she asks him what he has seen he replies that he has seen a dog. The name of the dog is “old Tim Johnson” which both Jem and Scout know.

“Whatcha looking at?”
“That old dog down yonder,” he said.
“That’s old Tim Johnson, ain’t it?”
“Yeah.”

The dog’s owner is Mr. Harry Johnson who lived in the southern edge of the town and was a bus driver. The dog was liver-colored. After seeing the dog they both ran back home to tell Calpurnia that the dog was not well and was behaving strangely. Finally Calpurnia sees the dog and notices it has developed rabies. She calls Atticus who kills the dog with a single shot much to the shock of his children.

I love the section where Atticus takes out the old dog, it is amazing to see the way that Scout thinks so little of him at the beginning of the chapter.  He doesn't "drive a dump truck," he isn't "the sherriff," he just works in an office and goes about his business.  He isn't tough or dangerous or really much of anything that Scout could brag about.

Then old Tim Johnson (yeah that was the name of the dog) gets rabies and decides to put Atticus' children in danger.  Atticus raises a rifle and puts a round through the dog's head from quite a distance and without any hesitation.

Suddenly Scout learns that her father is a "dead shot" and perhaps he isn't quite such a weenie after all.

At the beginning of chapter 10, Scout and Jem complain that their father is talentless and boring. One Saturday, the children notice a rabid dog staggering down the street, and Calpurnia informs the community to stay inside. When Heck Tate and Atticus arrive on the scene, Sheriff Tate hands his rifle to Atticus, who reluctantly takes the weapon. Against his will, Atticus reveals that he is an expert marksman by shooting and killing the rabid dog in one shot. Jem and Scout are astonished and awestruck by Atticus's marksmanship, and Miss Maudie proceeds to call him by his former nickname, "One-Shot Finch."

Miss Maudie then explains to the children that Atticus is a civilized man who would never brag about his God-given talents. According to Miss Maudie, Atticus refused to harm any living being once he realized that God had given him an unfair advantage over most living things. After witnessing the spectacle, the children discover Atticus's amazing talent and Jem learns the importance of conducting himself like a humble gentleman. The children also learn that Atticus is only willing to physically harm another being when it threatens anyone who cannot defend himself.

Later on, Scout recalls Atticus shooting the rabid dog just before Tom Robinson's verdict is read. By associating the mad-dog scene with the trial, Lee emphasizes Atticus's moral compass and connects the two scenes. One can interpret the rabid dog to symbolically represent Maycomb's ugly, dangerous prejudice, which is exposed during the Tom Robinson trial. Atticus using the sheriff's rifle symbolically represents his defense of Tom and his attempt to protect his innocent client.

The mad dog scene also parallels Atticus's lesson regarding mockingbirds. According to Atticus, it is okay for his children to kill menacing blue jays but it is a sin to kill innocent mockingbirds. The rabid dog metaphorically represents malevolent blue jays and other hostile, threatening beings. Overall, the mad dog scene characterizes Atticus as a humble, civilized man and underscores the theme regarding the importance of protecting innocent beings.

In this chapter, Atticus very reluctantly kills a rabid dog, Tim Johnson, with one shot. He doesn't want to do it, but rises to the circumstance. As a result of this incident, Scout and Jem learn that their father was once an expert marksman. They admire him more than they did before, because they recognize being a good gunman is a talent the rest of the community considers manly, brave, and admirable.

However, the real lesson they learn is that Atticus has his own moral compass, separate from that of the larger community. He doesn't like to kill things, especially with a gun, as he shoots so well that it gives him almost total control over other living creatures. He doesn't want to take advantage of his power. And just because the community approves of it, doesn't mean he is going to brag about his marksmanship or use it as way to raise his own esteem in their eyes. This way of living offers insights into his defense of Tom Robinson.

The children learn that one can both live in a community as a part of it and hold to one's own moral standards. As Jem says at the end of the chapter, his father is a real gentleman—gentleman in both senses of the word: a polite, dignified man of a certain social class, but also a genuinely gentle man. Or as Miss Maudie puts it to the children, he is "civilized." The children learn from their father's example and Miss Maudie's words that being civilized means not taking advantage of what you have to hurt others:

If your father’s anything, he’s civilized in his heart. Marksmanship’s a gift of God, a talent—oh, you have to practice to make it perfect, but shootin’s different from playing the piano or the like. I think maybe he put his gun down when he realized that God had given him an unfair advantage over most living things. I guess he decided he wouldn’t shoot till he had to, and he had to today.

Scout begins narrating Chapter 10 by discussing the perception she and Jem have of their father. Prior to the dog incident, they viewed him as an old man. That Atticus was a man of words did not impress them much; they felt he was not a man of action and therefore not as "manly" as the other children's younger fathers:

Atticus was feeble: he was nearly fifty. When Jem and I asked him why he was so old, he said he got started late, which we felt reflected upon his abilities and manliness.

Our father didn’t do anything. He worked in an office, not in a drugstore. Atticus did not drive a dump-truck for the county, he was not the sheriff, he did not farm, work in a garage, or do anything that could possibly arouse the admiration of anyone.

When Atticus forbade Scout from fighting, she believed this was another sign of weakness and made her look weak to her peers.

Miss Maudie offers some encouragement. She explains how well Atticus can construct a will, how well he plays checkers, and how he can play a Jew's Harp. But none of these things impress Scout.

When Atticus shows some physical ability, shooting Tim Johnson (the dog) with one shot, the children's perception of him changes. For the first time, they see that he can be a man of action. Following the dog incident, Scout wants to brag about it at school. But Jem realizes that Atticus would not want them to brag. As Miss Maudie said:

I think maybe he put his gun down when he realized that God had given him an unfair advantage over most living things. I guess he decided he wouldn’t shoot till he had to, and he had to today.

Atticus has no need to brag or show off. He uses his abilities when necessary. Jem, being older, begins to realize that Atticus' humility and character are as admirable as scoring touchdowns. Being a man of words and character is as admirable as being a man of action. He says, "Atticus is real old, but I wouldn’t care if he couldn’t do anything—I wouldn’t care if he couldn’t do a blessed thing." It seems that now, Jem is proud of who Atticus is; no longer wishing that Atticus was a younger, bragging father.

The dog, Tim Johnson, is referred to as "the pet of Maycomb." It would be a stretch to suggest that the dog is a symbol of Maycomb. But an argument could be made that an old dog, sick with a distorted perception of things is analogous to an old town, sick with racist traditions.

In both situations, Atticus Finch is forced to do something that he would rather not do but responsibly carries out his assignment despite his reluctance. When Judge Taylor tells Atticus to defend Tom Robinson, Atticus is aware that he is faced with an unwinnable case and that the trial will force his family into the town's spotlight. Atticus even explains to his brother that he had hoped he would never have to take a case like this but is going to follow through with his assignment. However, Atticus follows his conscience and does the right thing by defending Tom to the best of his ability. Atticus's difficult decision to defend Tom Robinson is similar to when Sheriff Tate hands him the rifle and instructs him to shoot the rabid dog. Despite his reluctance, Atticus knows that he must do the right thing and kill the rabid dog in order to protect his family and community. In both situations, Atticus is uncomfortable and reluctant to take action but does the right thing by following through with his given assignment.

The rabid dog also symbolizes the town's ugly racism. Similar to the mad dog, Maycomb's racism is dangerous, imposing, and harmful. By aiming the rifle at the rabid dog and shooting it, Atticus symbolically takes a stand against the town's racism, which is exactly what he does during the Tom Robinson trial. Unfortunately, Atticus cannot defeat the town's racism during the trial as Tom becomes the victim of racial injustice.

This is a very good question. The two events are connected in the sense that on both occasions, Atticus is forced to do something that he would have preferred not to do. Concerning the Tom Robinson trial, Atticus told his brother, Jack, that

"... John Taylor pointed at me and said, 'You're it.' "

Judge Taylor appointed Atticus to take the case and, though he could have turned it down, he--like Judge Taylor--realized that he was the only man for the job.

The same can be said for the situation with Tim Johnson. Atticus hated guns, and he was not proud of his past marksmanship skills. But Sheriff Tate

... almost threw the rifle at Atticus...

leaving Atticus with little choice in the matter. This time, it was Sheriff Tate who came to Atticus because he knew he was the right man for the job.