Also, on why the interest needs to be paid as well to consider the debt fully paid and not just the principal, beyond the obvious "honor the contract as stated" thing: Time is money, and investments made in one place can't simultaneously be invested in another.  If the money is not paid back, not only is that obviously robbing the person of their initial principal, but it's robbing that person of the gains they could have potentially made by investing the money elsewhere, but instead invested there.  That interest represents the time that the money they invested was tied up in their investments and not available to them, and guaranteeing an interest rate over that period makes you just as responsible for repaying that interest as the principal itself.

This applies to pretty much every other guaranteed interest rate lending nonsense here too.