[PsYcNeT]

Because 'Murica is rooted in a strong revenge fantasy exacerbated by the need to prove how big your peepee is.

So, if you let an intruder off with a warning shot and let them get away, obviously they will be so prideful as to hold a long-term grudge (because you would, right?) and come back at a later date to murder your family. Best shoot the bugger where he stands and get to be a big man in the process.