There was a case years ago in NYC with a big fat schizo lady named Eleonor Bumpers. She was coming at the cops with a kitchen knife. They should have just shot her in the legs and she would have gone down like a hippo with exhaustion....but when one is in that situation you do whatever it takes to defend yourself. Also, the question is, how many feet away is the person with the knife?
2 to 5 feet is preety close