In my estimation, Bob Ewell is the person who would best be described as mad or crazy in Harper Lee's To Kill a Mockingbird. (And while Boo is reclusive, he is not insane; he has been emotionally damaged by his father, but he is not violent, and he is very much aware of the difference between right and wrong, as seen at the end of the novel.)
Bob Ewell, we discover at the story's beginning, has accused Tom Robinson of raping Ewell's daughter Mayella. The bruising on her body, however, is from someone who is left-handed. Atticus has Ewell sign his name, and he is left-handed. It is not until Tom Robinson takes the stand that we realize he could not have smacked Mayella across the face with his left hand:
Tom Robinson reached around, ran his fingers under his left arm and lifted it. He guided his arm to the Bible and his rubber-like left hand sought contact with the black binding. As he raised his right hand, the useless one slipped off the Bible and hit the clerk's table.
It becomes obvious that not only did Ewell beat his daughter because (as we learn) she kissed a black man, but he is so twisted that he has no sense of guilt in wrongly accusing an innocent black man who, if found guilty, would die for raping a white woman.
After Tom is found guilty, Ewell threatens Atticus and spits in his face on the street. Miss Stephanie notes:
Atticus was leaving the post office when Mr. Ewell approached him, cursed him, spat on him, and threatened to kill him.
However, what shows Ewell's madness even more so is his attack of Scout and Jem: only a truly twisted and evil man would attack children, and only because he was mad at Atticus for defending a black man—and probably also because he showed Ewell to be a liar on the stand.
When Ewell is killed at the end, Atticus believes Jem did it. Heck Tate explains that it was not Jem, but Boo Radley, who had been watching the children and saved their lives. His explanation takes a while to set in with Atticus who believes Heck is simply trying to protect Jem because he is Atticus' son. Heck is actually trying to protect Boo, who "with his shy ways," would suffer greatly in the public spotlight for saving the children's lives. Heck describes Ewell's violent and murderous actions.
He'd flung Jem down, he stumbled over a root...
This shows Ewell's intent to do harm. That Boo saved them infers that they were in danger. Heck says that Ewell is dead—and it was Ewell's fault that Tom was dead. He infers that Ewell was in the process of also trying to commit murder:
I never heard tell that it's against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what [Boo] did...
Heck explains that Boo has taken care of a threat to the community—describing Boo as...
...the one man who's done you and this town a great service.
All of these examples show that Ewell was hateful and dangerous—beating his own daughter, and trying to kill Atticus' children shows that Ewell was insane.
Bob Ewell and Mayella Ewell are pretty close contenders for the title of the "most mad" in To Kill a Mockingbird. Madness suggests craziness, and in this piece, both Bob and Mayella also demonstrate madness as in anger.
Bob Ewell's pursuit of children in an effort to get back at Atticus in chapter 28 has to be the most crazy or mad act of an adult throughout the book. Before this act, Bob Ewell knowingly accused Tom Robinson, an innocent man, of raping his daughter when in fact, Bob probably was beating or taking advantage of his daughter himself. This is complete craziness. Bob feeds his craziness by being a drunk, something that is obvious when Scout feels his dead body and smells the whiskey.
Mayella's performance on the stand makes her a good second place for the most mad. She lied under oath, was caught by Atticus, and then went back to her original story. This drama queen derserves an award, but only behind her dad's performance.