Following Bob Ewell's attack, Sheriff Tate confirms that Bob died after falling on his own knife. However, Atticus does not believe Sheriff Tate and assumes that Jem is responsible for murdering Bob Ewell after listening to Scout's description of the attack. Atticus tells Sheriff Tate,
If this thing’s hushed up it’ll be a simple denial to Jem of the way I’ve tried to raise him. Sometimes I think I’m a total failure as a parent, but I’m all they’ve got. 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 (Lee, 277).
Essentially, Atticus believes that Jem should take responsibility for murdering Bob Ewell because it is the morally right thing to do, and he fears that his children would have a negative perception of him if he were to cover up Jem's involvement in the murder.
Throughout the novel, Atticus has tried his best to teach his children how to behave with honor and integrity. If Atticus were to lie about Jem's role in Bob Ewell's death, he would be doing his son a disservice, and his children would view him as a hypocrite. Atticus initially wants Jem to take the blame for Bob Ewell's death for the same reason he chose to defend Tom Robinson. Atticus simply wants to be able to live with the choices he makes in life and to have his children view him as a man of integrity. Eventually, Sheriff Tate indirectly tells Atticus that Boo Radley was responsible for Bob's death, and Sheriff Tate then explains why he refuses to inform the community of Boo's heroics. After listening to Tate's reasoning, Atticus accepts the fact that Sheriff Tate is doing what is best for their reclusive neighbor.