Bob Ewell. Atticus believes that Jem has killed Ewell; however, Mr. Tate says, "Jem never stabbed Bob Ewell." Atticus still believes that Jem has killed Ewell, but the sheriff contradicts Atticus. He insists that Ewell fell on his knife in the scuffle with Jem, contending that "Scout was too scared to know exactly what went on."

The truth is that Boo stabbed Jem as he came to the defense of the children. But Sheriff Tate sees no reason to involve Boo "with his shy ways," as Boo could not handle the attention that would result from an investigation. Instead, they can quiet the whole affair by "let[ting] the dead bury the dead" and saying that Ewell fell on his knife. Atticus agrees because Scout suggests involving Boo would be "like killing a mockingbird."

Atticus is convinced that Jem killed Ewell in self defense. To Atticus, this seems like the only possibility. He gets in an argument of sorts with Sheriff Tate over Jem's guilt. Atticus is so convinced on this idea of Jem protecting Scout from Ewell that he believes that any other explanation would amount to a "cover up." It's clear that Atticus, who as Heck notes is under tremendous strain, is not thinking clearly. His determination that Jem should tell the truth, that he cannot start life with this "hanging over him," is less about Jem than it is about his own emotional well being. As he explains to Tate,

Before Jem looks at anyone else he looks at me, and I’ve tried to live so I can look squarely back at him...if I connived at something like this, frankly I couldn’t meet his eye, and the day I can’t do that I’ll know I’ve lost him. I don’t want to lose him and Scout, because they’re all I’ve got.

Tate is impatient with Atticus because his contention that Ewell fell on his own knife, while not strictly true, is meant to protect not Jem and Atticus, but Ewell's real killer, Boo Radley. Heck knows the attention Boo would get from the public if the truth got out would mortify him. When Atticus comes to realize that Boo, not Jem, is Ewell's killer, he relents.

In a way, Ewell did kill himself, even if he did not fall his knife as Tate claims. As Tate says, Ewell was the kind of man "you have to shoot before you can say hidy to ‘em," and his death was brought about by his own actions.

Chapters 29 and 30 of To Kill a Mockingbird present the aftermath of Bob Ewell’s nighttime attack on the Finch children as they walk home from the school pageant. After an unidentified person intervenes and takes the children home, the doctor sees to them, and Sheriff Tate comes to the house. Scout is awake, but Jem is still unconscious. During the attack, Ewell was stabbed and died. It turns out their savior was Arthur “Boo” Radley, whom Scout meets for the first time.

In chapter 30, as Scout, Radley, Tate, and Atticus sit on the porch, the sheriff and Atticus discuss how to proceed. Atticus tells Tate that Ewell was killed through an act of “self-defense,” assuming that Jem stabbed him. He further expects that Jem will be arrested and tried, but not convicted. Tate counters that Jem did not do it; rather, Ewell accidentally stabbed himself. Atticus is displeased to think that Tate is suggesting that they conceal Jem’s role, and he then realizes that Tate knows that Radley was responsible.

Tate explains his decision not to pursue the matter further based on all the unwelcome publicity it would generate for Radley. He reasons that Ewell deserved his fate, given his numerous lies and violent acts. He tells Atticus that Radley has “done you and this town a service.” Had Radley not intervened, Ewell might have killed the children.

Heck Tate's first and foremost duty was to protect Boo Radley. But because Atticus seemed convinced from the outset that his own son committed great crime and was ready to see him fairly tried. I think a secondary reason for Heck to present this argument was to protect Jem. Obviously Jem was a victim, but knowing Ewell was dead, Atticus jumps to conclusions about how it was done. I think Atticus comes to believe that there was a struggle that resulted in Jem's injury, but that Jem was the victor who actually took the life of Ewell. Tate, knowing better than to incriminate himself or Boo tried to keep quiet and not discuss much with the ever-moral and just attorney Atticus. Atticus would have acted on the evidence trying to do the morally right. Tate has to leave him with the "let the dead bury the dead this time Mr. Finch."

After Jem and Scout are so viciously attacked, Atticus tells Heck Tate that Bob Ewell was "out of his mind" to do such a thing. Tate disagrees. He sees Ewell as a man who was totally depraved:

[Ewell] wasn't crazy, mean as hell. Low-down skunk with enough liquor in him to make him brave enough to kill children. He'd never have met you face to face . . . . 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, so do you think he'da met you to your face in daylight?

According to Heck Tate, Bob Ewell was a vicious coward who was worthless as a human being:

Mr. Finch, there's just some kind of men you have to shoot before you can say hidy to 'em. Even then, they ain't worth the bullet it takes to shoot 'em. Ewell 'as one of 'em.

Sheriff Tate was an excellent judge of character.

In chapter thirty-one of To Kill a Mockingbird by Harper Lee, Jem is in bed recovering from the attack by Bob Ewell, and Scout, Atticus, Boo Radley, and Sheriff Heck Tate are on the Finches' front porch. Atticus is convinced that Jem killed Bob Ewell and begins to prepare his son's defense in his mind. Heck Tate, however, knows exactly  what happened and tells a different story in an attempt to spare Boo Radley from unnecessary attention.

In the sheriff's version of events, no one killed Bob Ewell.

Mr. Tate flicked open the knife. “It was like this,” he said. He held the knife and pretended to stumble; as he leaned forward his left arm went down in front of him. “See there? Stabbed himself through that soft stuff between his ribs. His whole weight drove it in.”
Mr. Tate closed the knife and jammed it back in his pocket. “Scout is eight years old,” he said. “She was too scared to know exactly what went on.”  

Of course, Tate knows exactly what happened. He understood that the reclusive Boo Radley killed Bob Ewell in order to save Jem's life; Atticus takes a little longer to realize this truth. Even Scout seems to understand when Tate makes it clear why he would be willing to tell this lie.

There is no doubt that Boo Radley would not be found guilty, in a Maycomb county courtroom, of any crime for what he did to Bob Ewell. In fact, it is likely that such a trial would turn Boo Radley into some kind of a hero--something both men want to avoid if they can. They understand that people in town are likely to treat the reclusive man as a hero, something  he just does not have the temperament to endure; too much attention would kill him.

Finally Atticus sees the truth and realizes the sheriff is correct; but now he has to worry about setting the proper model (example) for Scout. She does, indeed, seem to understand what has happened and why they will not be telling the truth about Boo Radley killing Bob Ewell. She understands that Boo is a kind of mockingbird, someone who does nothing to hurt anyone and tries only to help where he is able. 

Because Atticus embarrassed Bob and Mayella Ewell in the courtroom trial of Tom Robinson, Ewell attempted to get back at Atticus by harming Scout and Jem.  It was a cowardly thing to do, but luckily, the bravery of Boo saved the kids.  Atticus decided to go along with Heck Tate's story about Bob Ewell falling on his knife to protect Boo Radley.  Boo had just saved Scout's life, and to Atticus and Heck Tate, Boo's life had been hard enough without answering questions or being arrested or prosecuted for the death of Bob Ewell.  Boo was an innocent who symbolized a mockingbird in the book.  Mockingbirds don't do harm, and therefore, should be protected.  Boo was a kind, gentle soul who had hardships to bear because of his family.  The publicity of the death would probably have caused a lot of anguish and problems for Boo who had been kept hidden away for years in the Radley house.  Atticus and Heck Tate realized that what happened to Bob Ewell was justice in a sense.  His evil, racist accusations about Tom Robinson were unjust.  His attempt at revenge towards Atticus showed what a horrible, violent person he really was.  In addition, neither Atticus nor Heck Tate wanted to put Maycomb through another trial involving innocent people.

After Bob Ewell attacks Jem and Scout and Boo Radley saves them, the sheriff, Heck Tate, insists to Atticus that Bob Ewell fell on his knife and killed himself. Atticus, however, denies this, for he thinks he knows exactly what has happened. He believes that Jem killed Bob Ewell either accidentally or in self-defense. Atticus also thinks that Tate is trying to cover that up so as to get Jem off the hook, but Atticus doesn't feel it would be right to brush such a thing under the rug. Jem is, after all, the son of a prominent attorney and politician, and covering up such an act, even if it were an accident or self-defense, would not look good.

Atticus, however, is wrong. Tate knows that Jem did not kill Bob Ewell. He also knows that Ewell didn't fall on his knife. Boo Radley killed Ewell as he was rescuing Scout and Jem. Tate, however, wants to cover that up, not because he fears legal repercussions for Boo (as Ewell was killed in the midst of trying to harm two children whom Boo was defending). Rather, Tate realizes that the people of Maycomb will flock around Boo with praise and gratitude and, more generally, attention if they find out that he saved Scout and Jem. Boo could not handle that. He is too much of a recluse, so Tate wants to spare him the attention. Therefore, he declares that Bob Ewell fell on his own knife, and Atticus, finally understanding, goes along with the story.

Atticus begins fretting about Jem.  He thinks that Jem stabbed Bob Ewell.  He is already thinking about an upcoming court case with a self defense plea.  Heck Tate tells Atticus that Jem did not stab Bob Ewell with a knife.  Atticus misunderstands Heck Tate.  He thinks the man is trying to spare his son a trial.  He tells Heck that he does not want to deny the truth.  Atticus does not want to cover up his son stabbing and killing a man.  

Heck tells Atticus that Bob Ewell fell on his own knife.  He killed himself.  Atticus insists that he does not want what Jem did covered up.  Heck protests again and even offers proof.  Atticus tells Heck that he needs to be truthful in all ways.  He cannot protect his son.  He says that he "can't live one way in town and another way in [his] home" (To Kill a Mockingbird, Chapter 30).  

Insistent, Heck shows Atticus proof.  He pulls out the knife.  He acts out what had happened.  Finally, Heck shouts at Atticus.  He tells him that he is not trying to protect Jem.  Instead, he is trying to protect Boo.