... but then I would have to kill you

I could tell you but then I would have to kill you. .......But where does it come from?

It was a line in American Psycho . There is also a site that attributes the phrase to Robert Hanssen (a spy), who was meant to have said it back in the 1990s. www.crimelibrary.com/ terrorists_spies/spies/hanssen/6.html