Familiar
O. Henry gives us a sympathetic narrator like Sam, someone who talks to us like an old friend and seems to have a tone of casual ease with us. The crime is so matter of fact, and the telling of it is familiar and strangely comforting. This comes about both from the writing style itself, such as using the phrase "Bill and me" (3) instead of the more grammatically correct "Bill and I." But it also arises in O. Henry's descriptions, such as when Sam "lighted my pipe and sat down on a mossy bank to await developments" (72). You can almost imagine settling down on the rock with him to listen to him unfold his tale. There is no true hardened criminal here. Bill and Sam are on the wrong side of the law, no doubt, but in a sort of gentle and sympathetic way. Sam's story seems like something your weird old uncle might tell at a family gathering.
These aspects of Sam's character are important because they help keep the tone light and satirical, which is exactly what O. Henry wants. He wants his audience to smile while he's making his points, something that's normally tough to do in a story about child abduction. A familial tone makes it bubbly and buoyant. There's some bite there (poor old Bill), but it doesn't interfere with the sense that this is a comedy first and foremost.