The advice the warden gives Jimmy Valentine is brief.
“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.”
At the time the warden's advice makes no impression on Jimmy at all. He is already planning to recover his specialized burglar tools and resume his life of crime. His reply to the warden shows his cockiness as well as his policy of never admitting to anything.
“Me?” said Jimmy, in surprise. “Why, I never cracked a safe in my life.”
O. Henry's whole purpose in writing that interview between the warden and Jimmy was to introduce the idea of reformation, which is what the story is all about. Jimmy pays no attention to the warden's advice, but oddly enough he ends up doing exactly what the warden advises. He decides to "live straight." His motive for reforming is falling in love with Annabel Adams, a very pretty and proper small-town girl in Arkansas. She would be shocked, horrified, and repelled by Jimmy if she knew about his past, including the fact that he had served time in more than one prison. Jimmy makes a decision to become worthy of this angelic woman by going straight. It may be that the warden's advice somehow lingered in his unconsciousness and found its way into his consciousness at this critical time. Jimmy remembers especially: "You're not a bad fellow at heart. Stop cracking safes and live straight." Maybe he isn't such a bad fellow at heart. Maybe he really could live straight! He writes a letter to a friend in which he tells him:
Say, Billy, I've quit the old business—a year ago. I've got a nice store. I'm making an honest living, and I'm going to marry the finest girl on earth two weeks from now. It's the only life, Billy—the straight one. I wouldn't touch a dollar of another man's money now for a million.
For a few bad moments at the end of the story, Jimmy's reformation seems in serious jeopardy. But everything works out all right in the end, proving that the warden's advice was not only sound but prophetic.
O. Henry, who always wrote under a pseudonym to hide his identity, spent several years in an Ohio penitentiary. He may have had a similar interview with the Ohio warden when he was released. O. Henry is being utterly sincere when he writes that the only life is the straight one. It may not be as lucrative and flashy as a life of crime, but it has all kinds of amenities that make it much more valuable. O. Henry contrasted the straight life and the crooked life in another of his best stories, "After Twenty Years."