At the beginning of the story, Tom Walker takes a shortcut to get home more quickly. The shortcut takes him through a swamp and is described as "an ill chosen route." The first example of it being ill-chosen is given when the author describes how difficult and dangerous it is for Tom to traverse the swamp. The swamp is "thickly grown" and full of "pits and quagmires, partly covered with weeds and mosses." This makes the shortcut through the swamp extremely difficult to traverse. It is also dangerous because Tom could easily fall and break an ankle, or worse. The swamp is also so overgrown that even at midday it is "dark," making it even more dangerous because it is difficult to see the "pits and quagmires."
The second example of the shortcut being an ill-chosen route is provided when the author implies that the shortcut actually made Tom's journey longer rather than shorter. We are told that Tom "had long been picking his way cautiously through this treacherous forest." We are also told that "it was late in the dusk of evening when Tom Walker reached the old fort." These references to time suggest that this shortcut actually lengthens Tom's journey home and thus by definition is an ill-chosen shortcut.
The third and most important example of Tom's shortcut being ill-chosen can be found in the form of the "great black man" who Tom meets in the swamp. This man has a face that is "begrimed with soot, as if he had been accustomed to toil among fires." This man is of course the devil. He offers Tom a great treasure in exchange for his soul. Tom eventually agrees to the deal and thus seals his own damnation. This certainly suggests that the shortcut was an ill-chosen route.