Seeing as how this went down the path I kinda thought it would:
How anybody reacts to such an event is a summary of the totality of the circumstances and the individuals involved. What we have here is a very narrow posting of circumstances which, of necessity, do not include such things as tone of speech, posture, body language, and any number of other cues we might be exposed to if we were actually on the scene.
Therefore, you cannot say that any given response, especially one which might involve the use of force (deadly or not), is truly warranted. Any response MUST include caveats, then.
Words screamed in the heat of passion, even clearly stated as "I'LL KILL YOU!" may not actually warrant an immediate physical response at all. Kids, for example, scream at each other all the time...anybody who has ever had toddlers/young kids knows they can be VERY emotionally heated in their screaming threats. Yet we don't jump on the "SHOOT 'EM NOW!" bandwagon. Likewise older, larger people. Do they have the means at hand? Are they actually capable of presenting a serious bodily threat? Is there time/distance to attempt a less violent result?
A hippy-type dude approaching me in a grocery aisle asking me this question while eyeing what's in my cart would draw a different response from me than a 6'4" angry scar-faced dude holding a length of pipe in a threatening manner.
What do you do?
The answer is "react appropriately for the circumstances".
That SOUNDS like a cop-out. But it's really not. Because you can be assured that if you end up in court over the result, others WILL be judging your response to determine whether or not it was actually "appropriate for the circumstances" in the eyes of the law.