"If they didn't have a weapon loan them one of your kitchen knives."
That's called tampering with evidence/alteing a crime scene.
It is the action of a guilty person trying to hide something.
It is also rather easy for competent crime scene investigators to figure out.
In short - it is really lousy advice that should NOT be listened to.
If you feel justified in taking the shot based on the circumstances at hand do not try to alter the scene to make things even more favorable for yourself.
It just ends up making you look like a liar.