In "The Devil and Tom Walker," why was Tom set against being a slave trader for his end of the bargain?

Before the devil agrees to give Tom his treasure, he wants to ensure that the money will be used "in his service". To this end he suggests that Tom use the money to buy and operate a slaving ship. The narrator notes that while Tom was a morally questionable character even he wouldn't stoop to the level of being a slave trader. Tom refuses to become a slave trader because he considers the practice morally...

Before the devil agrees to give Tom his treasure, he wants to ensure that the money will be used "in his service". To this end he suggests that Tom use the money to buy and operate a slaving ship. The narrator notes that while Tom was a morally questionable character even he wouldn't stoop to the level of being a slave trader. Tom refuses to become a slave trader because he considers the practice morally objectionable. Ultimately, Tom and the devil reach an agreement that he will use the money that he gets from the treasure in order to set up shop as a usurer or money lender in Boston. Tom's refusal to become a slave trader ultimately serves two purposes in the story. This refusal provides a moment of wry humor in the story as the narrator jokes that even the despicable Tom Walker wouldn't willingly become a slave trader. The other purpose of this refusal is to attack the practice of slavery and the slave trade by casting it as the devil's preferred occupation and as being too horrible for Tom.