“Heck,” Atticus’s back was turned. “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. I don’t want to lose him and Scout, because they’re all I’ve got.”
– Harper Lee
To Kill a Mockingbird, Chapter 30. Atticus thinks it was Jem who stabbed Bob Ewell, in self-defense, but he refuses to have Sheriff Heck Tate hush it up. He taught his children to lead honest, decent lives, so he feels he cannot be party to spreading a lie and undoing everything he has tried to teach them.