Tom Robinson was falsely accused and convicted because of a racially corrupt justice system, and he knew an appeal would probably not overturn his sentence. He had nothing to lose by trying to escape from jail, and was shot in the process, probably by people who would have been all too eager to kill him in the first place even if he just sat there minding his own business. So no, questioning it wouldn't do any good, even if he was killed in cold blood. The "tacked on ending" brings closure to the Boo Radley story for the children. They spend the whole movie misunderstanding this mysterious person who they're afraid of, and it turns out he is protecting the children because he's really a nice person and wants to be their friend. He actually ends up protecting them against the deranged father of the rape "victim," who is so upset at this white man for insulting him and his daughter by calling them liars when he defends a black man -- who Ewell and most of the whites in the town wanted to be guilty just for being black -- that he takes out his anger on Atticus' defenseless children. In a way, Ewell's death brings a small measure of vindication to the dead Tom Robinson, and teaches the children a lesson or two. I think it's both a wonderful book and movie.