O. Henry portrays Jimmy Valentine as an essentially good man who has just gone down the wrong path in life. The warden takes an interest in him because he can see that Jimmy is not like the ordinary criminals the warden has to deal with. He tells Jimmy:
"Now, Valentine,” said the warden, “you'll go out in the morning. Brace up, and make a man of yourself. You're not a bad fellow at heart. Stop cracking safes, and live straight.”
The warden has seen hundreds of convicts, and he knows that Jimmy is, as he says, "not a bad fellow at heart." Jimmy's experiences in Elmore bring out the good in him. O. Henry seems to be suggesting that there are many young convicts who could be reformed if given the opportunities. Prisons don't reform inmates but only make them incorrigible. The ease and speed with which Jimmy reforms after falling in love at first sight show that he is basically good. When the little girl gets accidentally locked in the bank vault, we know that Jimmy is quite capable of freeing her. He even has his whole set of safecracking tools with him. We share his dilemma. Will he crack the bank vault and thereby expose himself as a criminal and lose everything he has gained through his reformation? Or will he just let the little girl die? We know he can't do that. Maybe some criminals would let the child die, but we know Jimmy too well by this time, and we are not surprised when he opens his suitcase and does a masterful job of breaking into a supposedly burglar-proof bank vault. We feel it is something we would have to do ourselves under the same circumstances if we had the expertise and the equipment. Jimmy has at least two strong motivations. One is to save a child. The other is to do what his loved one asks him to do.
Annabel turned to Jimmy, her large eyes full of anguish, but not yet despairing. To a woman nothing seems quite impossible to the powers of the man she worships.
“Can't you do something, Ralph—try, won't you?”
How can he resist such a plea, even though he thinks it means losing her? He has gone straight for her sake. If he is truly straight, then he must behave as any decent man would. It is ironic that he uses his safecracking genius for a good purpose, and it is doubly ironic that Ben Price, who is here to arrest him for those three bank jobs in Indiana, lets him go free because he saw him cracking a safe in a bank.
Jimmy is a safecracker, but O. Henry shows throughout that he is, as the warden says, a good man at heart. Everybody likes him--and they must like him because they sense that he friendly, cheerful, kind and generous. His reformation comes about just in time. If he had spent more years behind bars like a caged animal, it would have turned him into a bitter, antisocial, and potentially dangerous felon.