The introduction to the Gutenberg on line edition of this story states that for Washington Irving, the true devil was not a supernatural being but slave traders and witch persecutors. Therefore, Tom's refusal to fit out a slave ship and become a slave trader reflects Irving's abhorrence of the slave trade. Tom may be corruptible, but slavery, "the black traffic," as it is called in the story, is beyond the pale of even a very bad man who has sold his soul to the devil:
This, however, Tom resolutely refused: he was bad enough in all conscience; but the devil himself could not tempt him to turn slave-trader.
In the story, we also learn that the "black man," the devil, persecutes the religious groups that fought slavery, such as Quakers.
Tom's horror at the slave trade reflected the view of abolitionist groups such as the Quakers, certain Methodists, and the Transcendentalists, among others, that slavery was an intolerably evil institution that must be abolished. These groups, on the whole, believed slavery to be both a social and a moral evil. Quaker John Woolman, for example, worked tirelessly to eradicate slave-owning among Quakers (some did own slaves in the eighteenth century—at least until Woolman persuaded them otherwise). Woolman, like others, based this stand both on how slavery created a sick society of overly pampered whites and overly abused blacks, as well as basing his stand on the violation of the Christian command that one should love one's neighbor as oneself.