From the great Hitchcock film, Dial M for Murder: "[P]eople don't commit murder on credit."
If you haven't seen it, I recommend you watch it. Woody Allen's 2005 film, Match Point, seems to have drawn some inspiration from this Hitchcock film. I usually like Woody Allen, but for some reason, I didn't like Match Point. Most of my friends, however, did, so don't let me stop you from seeing it.