What explanation does Atticus give for Bob Ewell's attack? Chapters 29-30.
As Sheriff Tate looks at the remains of Scout's costume, he remarks, "This thing probably saved her life. . . . Bob Ewell meant business." Atticus's response to the sheriff's conclusion is, "He was out of his mind."
While Atticus acknowledges that Bob Ewell has desired revenge against him, he cannot believe that Ewell would stoop to harming his children, despite his sister's earlier remark that "[H]is kind'd do anything to pay off a grudge. You know how those people are" (Ch.23). More in accord with Alexandra's thinking, Sheriff Tate sums up Ewell as a "low-down skunk with enough liquor in him to make him brave enough to kill children." (Ch.29)
In this scene, the conundrum of whether man is innately good or if there is an intrinsic evil in the hearts of some people presents itself—Atticus takes one position on this, and Heck Tate the other. Since Sheriff Tate is more aware of the evil acts that men commit, he understands better the nature of Bob Ewell. He adds that Ewell's attack on the Finch children is not his first predatory act upon the defenseless. Tate argues,
He had guts enough to pester a poor colored woman, he had guts enough to pester Judge Taylor when he thought the house was empty. (Ch.29)
Before she leaves the room, Alexandra tells her brother that she had a feeling of foreboding earlier. She feels that what has happened is her fault because she did not warn her brother. But Sheriff Tate tells her not to blame herself because if people followed their feelings all the time, they would get nowhere. Then, he continues his efforts to persuade Atticus that Bob Ewell, who has had evil in his heart, fell on his knife that he meant to use on Atticus's children.
Atticus was more surprised than anyone present when Sheriff Tate reported that Bob Ewell was responsible for the attack on Jem and Scout. Atticus never actually expected Ewell to harm his children; however, he figured Ewell might eventually come looking for him. He knew that Ewell was unpredictable and full of hate. And more than that,
"He was out of his mind," said Atticus.
Sheriff Tate thought differently, calling Ewell a "low-down skunk."
Atticus shook his head. "I still can't conveive of a man who'd--"
"... I thought he got it all out of him the day he threatened me. Even if he hadn't, I thought he'd come after me."
Sheriff Tate was certainly more accurate about Ewell's character. Atticus, as usual, was willing to look on the good side of human kind, thinking even Bob Ewell wasn't evil enough to attack children.
Heck Tate is checking out Scout's costume during his investigation after the attack. He sees the wire mesh on Scout's costume. It had been sliced. He shows it to Atticus and explains that it probably was what had saved her life. He tells him that Mr. Ewell had meant to harm the children.
Atticus makes the statement that Mr. Ewell was just out of his head.
Heck Tate responds by contradicting Atticus and explaining that the man was a mean drunk and he was only brave enough to hurt a child. Following the trial Mr. Ewell had threatened Atticus, but instead he had gone after the children. This notion is very hard for Atticus to understand as he has always tried to look for the best in people.