After Bob Ewell is killed, Atticus and Heck Tate argue over how to handle the situation. Scout thought that Jem pulled Bob Ewell off of her and from this information, it is possibly that Jem may have stabbed Bob. Because Atticus is a model of honesty and integrity, he wants this event, trial and everything, to be out in the open. He doesn't want any suspicious thoughts (in Jem's mind or in the minds of others in town) to linger. Atticus just wants to do the right thing. What Atticus didn't, at first, understand is what Boo's role was in all of this and what an investigation might do to Boo.
However, Heck Tate realizes that is was more likely that Boo Radley was the one who pulled Bob away and stabbed him. Heck reasons that Boo did an honorable thing and to punish him (by making him a public spectacle) would be a sin:
“I never heard tell that it’s against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did, but maybe you’ll say it’s my duty to tell the town all about it and not hush it up.
Heck tries to persuade Atticus that it would be better to just say that Bob fell on his knife. Atticus realizes the ethics and logic of what Heck is arguing and finally agrees. Scout also realizes that putting Boo in the "limelight" would be like shooting a mockingbird.