So ... the police shoot you for moving your hand towards your waist? Pre-emptive strike-fear-or over ambition? What's wrong with waiting until you actually SEE a GUN? Terrible incident all around. Prankster should be tried for manslaughter at minimum. Officer... well there should be some repercussions for all involved.
Gernerally, that depends on the situation, and the knowledge the officer has at the time. When I first heard the story, I assumed that the guy was shot within a few feet from the officer (still not sure who did the shooting). However, like I said earlier, if the officer that shot the guy is the same person we see from the body came, that's a hell of a lot of distance. And distance and reaction correlate. IMO, at that distance, the officer (who I am assuming is in cover) had the luxury of waiting to determine if the guy actually was armed.