I've seen it happen a few times, not a triple homicide, but two friends where the lender gets angry and talks about revenge. It seems to stem from two factors:
1. The financial loss.
2. The lender being insulted when the borrower uses the lender's "jerk" behavior as a reason he's not paying.
There are many, many proverbs about not loaning money to friends, but people still do it. It rarely ends well.
Agree, but killing the debtor guarantees you'll never see the money. The old breaking a few bones technique works better and won't land you the death penalty if you get caught.