Irving uses his characterization of Tom and his wife to satirize greed, self-centeredness, and impulsiveness. Tom and his wife are so stingy that they won't share with one another, and glory in the other's misfortune (Tom's wife wants him to barter away his soul in order to receive financial benefits, and Tom doesn't seem to care when he finds out that his wife was most likely killed by the devil--he's more concerned about the items she "stole" from the house).
Later, when Tom becomes a usurer (a high-interest money lender), he cares nothing about his neighbors' and fellow church members' financial struggles; instead, he concerns himself with piling up as much money as possible for himself.
Irving's satirical tone toward and descriptions of the Walker, Old Scratch, and some of the town leaders (through the scene in the forest with the devil) demonstrate greedy, fickle human nature.
Irving's largest criticism of human nature arrives directly through the story's central figures: Tom Walker and his wife. Tom is a miserly man whose wife matches him in terrible character traits. Rather than working together as a loving couple, Tom and his wife are eager to cheat each other and to hoard their "wealth" (in one case, even a mere egg!) away from the prying eyes of each other. These two are so awful that when Tom's wife learns that he plans to sell his soul to Old Scratch in exchange for prosperity, she actually tries to beat him to it; instead, Tom's wife is brutally killed by the devil, an act which has little emotional impact on Tom. Although Tom consents to selling his soul and receives the riches he desired, he doesn't abandon his miserly ways. Tom tries to devise a plan to avoid "paying the piper"; he buries his dead horse upside down and saddled in hopes that he will be able to escape on it when Old Scratch returns to collect him. Even then, he is intent upon scamming others.
The characterization of these two horrible people seems to be quite a damning identification of the greediness of the human spirit, our selfish disregard for the wants and needs of others, and our obsession with the notion of scarcity.