Is the message of "The Devil and Tom Walker" about bad moneylenders, bad people meeting evil, or most people being too greedy to help others?
Your best choice is the third one. The story isn't just about the occupation of money-lending. Irving could have chosen another occupation and still told the story. The message of the story isn't about what happens when a person treats family members badly. I don't think Irving believed that the majority of people in the world didn't care about helping others, but the message of the story deals with what happens to people who are so greedy that they will go into business with evil people to further their own means.
Greed is one of the most important themes in the novel. Tom Walker "was not a man to stick at trifles when money was in view." Tom turns down the first offer of the devil because he's so greedy he doesn't want to share his wealth with his wife. As a money-lender, he claims to be a friend to those in need, but in reality, "In proportion to the distress of the applicant was the hardness of his terms." The neediest people were the ones with the harshest terms of paying the money back to Tom.
Because Tom lacks morals, he's willing to make a deal with the devil, the greatest personification of evil in the world, in order to become very rich. Selling his soul to the devil is not a problem for Tom until the devil comes to collect, and only then does Tom try to cheat the devil to save himself from hell. "Such was the end of Tom Walker and his ill-gotten wealth. Let all griping money-brokers lay this story to heart."
I think there is some truth in all of your statements, but perhaps the one that comes closet, for me, is the last. I don't know that Irving believed that "the majority" of the world was corrupt, but he was certainly seeing it as an ever increasing problem. Tom is not all, then, but many. Here is a quote that reflects the problem of wealth vs. poverty: "Tom was a "universal friend of the needy," even though "In proportion to the distress of the applicant was the hardness of his terms."
It is a theme Irving has dealt with before. As America grew, he personally became bewlidered by the destructive rapidity of change in American life, and not for the better. Slavery, greed, and American abundance not equally shared are themes you can find in almost all of Irving's narratives.
(Unfortunately, greed has only grown since Irving's time, a preying on the poor commonplace. Just yesterday I saw an ad offering to "put $1000-$5000" in your bank account at an interest rate of 99.25%!)
I think a good theme for the story is "We reap what we sow." If we only go around worrying about ourselves and what we can get out of a situation to benefit us, then we have truly missed the boat. We aren't here to be selfish, uncaring, and immune to others feelings. Everyone is our "Brother" no matter what race, color, or creed. We should watch out for our neighbor and try to make the world a better place instead of one filled with fear and hate. If we don't start creating a better world, we will suffer the consequences. The other saying is, "what goes around comes around." I would not want to see what happens if we don't start working together to find peace.