Revealing Boo’s part in the death of Bob Ewell would be a futile gesture which would bring no positive outcome, but would destroy the fragile character of Boo.
Boo Radley was acting in defense of Jem and Scout when he intervened in Bob’s attack on them as they left the pageant. Bob is found with a knife in his chest – most likely to be his own. It would be of no benefit to the community, Heck Tata and Atticus reason, to prosecute anyone for the crime. Bob sealed his own fate in attacking the children, and although the judgment that he “fell on his knife” may not be true, his death should be the end of tensions in Maycomb, rather than the continuance of them.
Both Boo and Bob are outsiders, but Boo brings happiness and comfort to those who acknowledge him. He is, like the mockingbird, an elusive, fragile, yet positive force in the community.