"I'll be your dumb decoy duck" Why does Holly finally agree to help Calloway?.
"A person doesn't change because you fin out more", but they do. For a person like Harry Lime who has not even appeared in the picture yet, we make judgements about him according to what we know, Holly has known Harry as his best firend ofr 20 years but that has crashed down around him in his realisation that Lime is indeed as Calloway says "the worst racketeer who ever made a dirty living in this city". It is Lime's betrayal of Holly, hist twisted and cynical ideas of morality and Holly's underlying simplistic, yet honest belief n right and wrong which lead to him agreeing to Calloway's plan.
Holly has a clear picture of what Harry is to him. It is this vivid pciture that sets him on a wuest to "hunt down the sheriff who [is] victimising his best friend". Calloway must be in the wrng because Holly's Harry isn't like that. What Holly comes to realise is that he doesn't know Harry at all. How indeed, could such an honest and trusting man like Martins, be best friends with such a devil? When Holly finds out the truth he naturally begins to doubt himself. Something he was previously so sure of has fallen down around him. In some way, by agreeing to Calloway's plan he is taking a kind of revenge on Harry "for laughing at fools like [him] all the time." For making Holly believe he knew the "real" Harry.
"Again the devil took [Jesus] to a very high mountain and showed him all the kingdoms of the world and their splendour; and he said to him "all these I will give you, if you all down and worship me"" Matthew:4:8. Harry Lime's world is viewed from upon high, just like Satan's. People are "dots" and thus he can be detached from the flesh and blood reality of his victims. We know Harry's view of the world to be wrong and cruel. He is cynical and hopeless about mankind and so turns to crime. Holly's contrasting faith in people (including the faith he had in his best friend's innocence) is what also converts him to being Calloway's "duck".
