Why did Bill and Sam kidnap the boy in "The Ransom of Red Chief" by O. Henry?

Sam, the narrator of "The Ransom of Red Chief," inadvertently reveals he and Bill are unsuccessful con men always hoping to strike it rich. Sam and Bill are in an unfamiliar part of the country, and, in spite of all their prospecting for suckers, they have only six hundred dollars between them. This apparently is not enough for them to invest in any of the scams with which they are familiar. 

Bill and me had a joint capital of about six hundred dollars, and we needed just two thousand dollars more to pull off a fraudulent town-lot scheme in Western Illinois with.

Currently, the men are in Alabama. There is no way they could pull off anything like "a fraudulent town-lot scheme" where they are. They don't have the capital. A kidnapping seems to be the only feasible way of raising two thousand dollars quickly, and they pick Ebenezer Dorset's son because Dorset is the wealthiest man in the region. It is evident Sam and Bill have no prior experience with kidnapping. The two men must be bachelors who know nothing about controlling little boys; the ten-year-old who calls himself Red Chief has the worst characteristics of the worst little boys, as Sam and Bill discover soon after kidnapping him.

As Sam says at the beginning of his story,

It looked like a good thing: but wait till I tell you. We were down South, in Alabama—Bill Driscoll and myself—when this kidnapping idea struck us. It was, as Bill afterward expressed it, “during a moment of temporary mental apparition”; but we didn’t find that out till later.

Sam thinks kidnapping is a good idea, but Bill afterward called it "a moment of temporary mental apparition," or "aberration." Both men use bigger words than they understand in an attempt to make an impression on the yokels they deal with in their profession. In this respect, Sam and Bill are like the King and the Duke in Mark Twain's novel Huckleberry Finn. Sam and Bill do not anticipate running into a yokel who is smarter than they are. This kind of story was popular in America in the horse-and-buggy days. Readers enjoyed reading about how "city slickers" were outwitted by apparently ignorant and unintelligent country folk. 

The kidnapping scheme was the result of desperation, ignorance, bad planning, and inexperience. The resolution illustrates O. Henry's theme: Crime does not pay. The reader can understand how Bill and Sam gradually realize that they have gotten themselves into a mess and how painful it must have been to part with $250 of their meager capital. Sam and Bill still have to travel all the way to Southern Illinois if they are going to work their "fraudulent town-lot scheme," and they will still need over two thousand dollars when they get there.