In the words of Ezekiel we read this extraordinary warning:
“If (a man) fathers a son who is violent, a shedder of blood, who does any of these things (though he himself did none of these things), who even eats upon the mountains, defiles his neighbour’s wife, oppresses the poor and needy, commits robbery, does not restore the pledge, lifts up his eyes to the idols, commits abomination, lends at interest, and takes profit; shall he then live? He shall not live. He has done all these abominations; he shall surely die; his blood shall be upon himself.” (Ezekiel 18:10-13)
It seems odd to argue that we must not murder, comit adultery, oppress the poor, rob and be an idol-worshipper, but that we may nevertheless charge interest on a loan. Indeed, the medieval prohibition on charging interest was based entirely on the understanding that it was immoral.
Go read the whole thing. Something I've pondered rather a lot.