> "Would they steal stuff if they could afford it?"
What if the other person answers: "Yes. For the thrill of it."?
> "why don't people steal the free handkerchiefs at tables?"
Again, they could simply answer: "Some people do. Most people don't because the risk vs reward is not worth it."
I don't find this line of reasoning convincing because it requires that the other person accept your view of the world.
Someone who has fully internalized the "human nature is greed" indoctrination will not be convinced by you simply stating the opposite.
> Q: "why do "bad" people do "bad" things"
> A: "Bad people do bad things because they are bad people."
As materialists this statement sounds like nonsense to us but you have to try and put yourself in the mind of the person you are arguing against: to them this statement makes perfect sense, and because it already explains everything it would be a waste of time trying to think of other reasons.
More than that even: the mere fact that you are trying to find material reasons why bad people would do bad things is, in their view, you trying to justify and excuse those bad people doing bad things, which makes you a bad person too. This is how idealism works.