In Harper Lee's To Kill a Mockingbird, there are several examples of insanity. The first can be seen in the rabid dog, Tim Johnson. Tom Robinson is temporarily insane when he makes a break for the jail yard fence. I believe there is madness in Bob Ewell whose hate is so strong that he tries to kill Atticus' children. However, I do not believe that Boo Radley is insane. His development has been arrested (stopped) so that he acts like a child, but he knows the difference between right and wrong. The town "scold"(gossip), Miss Stephanie Crawford, likes to spread outrageous stories about Boo, but gossip is not based in fact. Boo has been abused and his spirit has been broken, but he is not insane.
Tim Johnson drives fear into the heart of the community as he staggers down the street. His disease is deadly and the people in the town have a genuine fear of him, locking doors and staying far away.
[Tim Johnson] walked erratically, as if his right legs were shorter than his left legs.
Calpurnia rushes the children inside and calls Atticus:
I swear to God here's a mad dog down the street a piece—he's comin' this way...
Atticus tries to get Heck Tate to kill the dog, but the sheriff knows that only Atticus can hit Tim—anyone else would miss. Heck notes that the dog is nowhere near death—he is still able to kill people if he isn't stopped. Atticus shoots the animal and the children are warned to stay away from the body: dead or not, the dog carries a fatal disease—and one of its effects is madness.
Tom Robinson tries to escape and is killed:
"Tom's dead...They shot him," said Atticus. "He was running...They said he just broke into a blind raving charge at the fence and starting climbing over. Right in front of them—"
Atticus had told Tom he felt they had a good chance at an appeal, but Tom can't face imprisonment or execution. He has no chance to succeed, but he goes berserk ("raving") in a split second: trying to climb the fence with the use of only one good arm. It is an insane attempt, and he is shot 17 times in the back.
After Ewell's attack, the sheriff looks at Scout's costume, damaged by Ewell's knife:
"Bob Ewell meant business," Mr. Tate muttered.
"He was out of his mind," said Atticus.
Tate disagrees, saying that Ewell was mean and drunk. But consuming large amounts of alcohol makes people temporarily insane. His hatred for Atticus's defense of Tom Robinson and his drinking drives him into a murderous frenzy, and he tries to kill Jem and Scout. He was mean, but at that moment, he was insane with hatred and alcohol.
Arthur Radley is a gentle man. Gossip paints a terrible picture of the elusive Boo, but he sews Jem's pants for him, and he brings Scout a blanket the night of the fire. He laughs at the kids' antics as they play outside. And it is Boo who keeps an eye on the children: the only one who knows they are in danger. He understands Ewell's intent. He stops Ewell, and carries the unconscious Jem home.
Atticus believes that Jem killed Ewell and that Tate's denial is to protect Jem. Tate tells Atticus he's worried not for Jem but for Boo; for Boo saved the children's lives, not Jem. Tate knows that if he does tell the town, people will descend on Boo...
...knocking on his door bringing angel food cakes...draggin' him with his shy ways into the limelight...To me that's a sin.
Had Boo been insane, Tate would not have protected him; but for the heroic thing he did, they say Ewell fell on his knife.
There are gothic elements in the story. A gothic element is usually something spooky or strange, dark or macabre. In this story, the first element of insanity is Boo Radley. The second is the rabid dog. The third is Bob Ewell.
Boo Radleyis the monster in the book. He is the man who doesn’t come out of the house, and who had a wild youth and stabbed his father in the leg with scissors. The children are both terrified and fascinated of him. Jem gives an “accurate description” of Boo Radley.
Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that's why his hands were bloodstained-if you ate an animal .raw, you could never wash the blood off. There was a long jagged sear that ran across his face; what teeth he had were yellow and rotten; his eyes popped, and he drooled most of the time. (ch 1)
The gothic element is strong in Boo, because he is a tragic but mysterious figure. Boo comes out a few times in the story, in secret, and the children come to appreciate him as a gentle friend. However in the beginning, all of the children in town are afraid of him and he is the victim of much town gossip.
There are other gothic elements in the story, including the fire and the mob that tries to go after Atticus. However, the best example of insanity personified is the mad dog.
Calpurnia's message had been received by the neighborhood. Every wood door within our range of vision was closed tight. (ch 10)
The mad dog shows up in February, which is not seasonal. Atticus comes to the town’s rescue, as he symbolically does with the trial. The rabid dog is a danger to everyone, and Atticus shoots it.
Bob Ewell is the other insane figure. Like the rabid dog Atticus faces down and shoots, Bob is out of control. At the end of the book he attacks the children with a knife, but Boo saves them. Bob Ewell's racism and ignorance was the real insanity.