In "To Kill A Mockingbird" explain Mr. Underwood's editorial. What is he trying to say? How does this tie in with the title?

Mr. Underwood, in a fiery judgment against the shooting of Tom Robinson as he escaped, declares that "it was a sin to kill cripples, be they sitting, standing, or escaping."  He likened Tom's death to the senseless slaughter of songbirds by hunters and children. He is basically saying that it doesn't matter that Tom was escaping; he was handicapped, and to shoot him was an awful and senseless atrocity, just as killing songbirds for sport is an atrocity. 

This relates to the title, because earlier in the book Atticus had told the children that to shoot the Mockingbird was "a sin".  To shoot an innocent bird, for no reason, that was just singing its song, was an awful sin. In essence, that is what happened to Tom-not only in his death, but also in his unfair conviction.