In short, the answer is "be careful what you ask for." In O. Henry's story, the narrator and his friend, Bill Driscoll, have decided to kidnap the son of a prominent local citizen in hopes that his father will fork over money the pair need "to pull off a fraudulent town-lot scheme in Western Illinois."
However, when they manage to get the boy, (who calls himself "Red Chief") they get much more than they bargained for. The child fancies himself the subject of one of his favorite games, cowboys and Indians. He fights them "like a welter-weight cinnamon bear."
Despite their disparity in sizes, the boy manages to best the men at every turn:
"He put a red-hot boiled potato down my back," explained Bill, "and the mashed it with his foot"
Later, the narrator is awoken by horrific screams from his companion:
I jumped up to see what the matter was. Red Chief was sitting on Bill's chest, with one hand twined in Bill's hair. In the other he had the sharp case-knife we used for slicing, bacon.
The men know they have made a terrible mistake and want to get "Red Chief" home as quickly as possible. But the boy doesn't want to go. He's having too much fun with his friends.
So, again, be careful what you wish for....As the poet Robert Burns says, "The best laid schemes of mice and men/Often go astray."
O. Henry based several of his best stories, including "The Ransom of Red Chief," on what might be described as the simple moral that crime does not pay. In addition to "The Ransom of Red Chief," this theme, or moral, or message can be detected in "After Twenty Years" and also in "A Retrieved Reformation." In the Jimmy Valentine story the author actually has the hero spell out the message in a letter to his friend:
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 tomorrow. It's the only life, Billy--the straight one.
In "After Twenty Years," O. Henry contrasts one man who led a straight life with another who made more money but was always on the lam and ended up being carted off to prison.
O. Henry treats the kidnapping plot in "The Ransom of Red Chief" as a comedy, but what those two crooks were doing was very serious. They were kidnapping a little boy for ransom. Their crime not only did not pay but cost them all the money they had.