Why is Atticus so insistent that Jem killed Bob Ewell?

In chapter 30, Atticus and Sheriff Tate are discussing Bob Ewell's death, and Atticus mentions that he believes Jem was responsible for murdering Bob in self-defense. However, Sheriff Tate disagrees with Atticus and insists that Bob Ewell fell on his own knife and died. Atticus initially believes that Sheriff Tate is attempting to cover up Jem's role in Bob Ewell's death and refuses to allow Tate to protect Jem. As a morally upright man, Atticus has tried his best to raise his children with integrity and does not want to let them down as a parent. Atticus explains his reasoning for not allowing Sheriff Tate to cover up Jem's involvement by saying,

"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" (Lee, 277).

Essentially, Atticus does not want his children to discover that he attempted to manipulate the law in any way to protect Jem, because that would make him a hypocrite like the other Maycomb citizens. Eventually, Sheriff Tate indirectly informs Atticus that Boo Radley was the person who stabbed and killed Bob Ewell during the struggle.