A rather inscrutable character, Boo Radley has a past that leaves much to conjecture. For instance, the story of Boo's having stabbled his father's leg and Mrs. Radley's screaming in the street that Arthur "was killing them all" is dubious because of its narrator, Miss Stephanie Crawford, the "neighborhood scold" and because Boo was merely taken to jail and then released into the custody of his family, who refused to send him to the state mental hospital, Bryce, in Tuscaloosa, Alabama.
After this incident, Boo becomes a recluse. Apparently, he watches out the window and observes Jem, Scout, and Dill in their surreptitious activities. Vicariously, then, he begins to participate in some of their actions; he mends Jem's torn pants, and places little gifts in the knothole of a large tree until his brother Nathan seals the hole. It is not until Boo is disturbed by Bob Ewell's attack upon the Jem that Boo appears to recognize the evil that men do, thus losing his ingenuousness. Having grown to care for the children with whom he has tried to interact, Boo rushes to help Jem and, in so doing, arrests Ewell in his reprehensible act, stabbing him to death.
Yet, Boo remains a "mockingbird," an innocent of society who intends no harm. Moreover, Sheriff Tate understands that it's "a sin to kill a mockingbird" as he explains what he will attest about the incident and telling Atticus,
"To my way of think', Mr. Finch, taking the one man who's done you and this town a great service an' draggin' him with his shy ways into the limelight--to me, that a sin. It's a sin and I'm not about to have it on my head..."
Therefore, Boo Radley loses his naivete of perception on mankind--if he even has it when he confronts and kills Bob Ewell; however, he does not lose his innocence of soul as he remains a "mockingbird."
He doesn't really, however you could interpret his killing of Bob Ewell (out of self defence) as a loss of innocence. This was only because Bob Ewell attacked Jem and Scout on their way home from a halloween pageant. After all he saves their lives.