Here's a paragraph written by Fred Reed that explains to some extent how cops don't have the time to fully identify threats and sometimes wind up shooting unarmed people. Other timed, they don't get to go home at the end of the shift -- or ever.
"Let?s drop the ?You are a cop? narrative. Instead, let?s try an experiment. In your living room, no adrenaline, no darkness, no danger, I will turn my back on you, holding in front of me in one hand a Day-Glo yellow plastuc banana and, in the other, a realistic plastic pistol. You, in calm, perfectly safe circumstances, will point a ?pistol? at me. Your finger will do fine. I will turn as fast as I can with one or the other in my hand. You have to shoot or not.
You will find, no matter how many times we try the experiment, that I can turn and fire (if I turn with the gun) before you can decide whether I have a gun or a Day-Glo banana. Try it in a dark alley." -- Fred Reed (fredoneverything.com)