Here's something from one of Saint Leo's sermons that strikes me as very pertinent today:
[T]he money-lender's trade is always bad, for it is sin either to lessen or increase the sum, in that if he lose what he lent he is wretched, and if he takes more than he lent he is more wretched still. The iniquity of money-lending must absolutely be abjured, and the gain which lacks all humanity must be shunned. A man's possessions are indeed multiplied by these unrighteous and sorry means, but the mind's wealth decays because usury of money is the death of the soul.
I wonder how the world might be different if we, as a culture, had taken this biblical principle seriously. I really do.