If you are unfamiliar with guns, and yet want an idea of why shootings by cops happen, let me suggest a few practical exercises that may shed light on the subject.
First (and for a variety of reasons I don’t suggest trying this should the occasion arise in real life), have a friend stand in front of you and point his finger at your chest as if it were a gun. Hold your hands at about shoulder height, as if he had said, “Hands up!” Tell him to shoot instantly if you move. Then, as fast as you can, without warning, try to knock his hand away.
He will never be able to pull the (imaginary) trigger before you hit his hand.
Next, get a second friend, or just pretend you have one at hand. Hold your hand at your side as you might comfortably hold a gun, and point it at the second friend, who we will assume to be a clerk at a convenience store. If you are right handed, have your other friend stand directly to your left, pointing his “gun” at you, ready to fire. He will be the cop. If he wants, he can say, “Drop it!” or whatever.
Now, turn to look toward him, simultaneously pointing your gun finger at him, as fast as you can, and shoot. You will find that, although he already has his “gun” aimed, he cannot react fast enough to shoot before you do.
Reflect that in real life, unless he is cold and experienced in gunfights (cops are usually neither) he will probably wait a split second to decide whether the bad guy has hostile intent. End of story.
Try a harder one. Point your gun, again at a natural waist level, and close to your body, toward the “clerk.” Have the cop stand directly behind you, say, ten feet away, with his gun aimed at you. Now, as fast as you possibly can, twist — don’t step — around and try to shoot him before he can shoot you. The results may surprise you. Note that, if the “cop” had his gun aimed at the middle of your back, he will probably find that it is pointed at the clerk when he pulls the trigger because, in twisting around, your body moved out of his line of fire.
Most criminals won’t have the presence of mind to try turning on you this way, but some will. If a robber is wired out of his mind on PCP or crank, he will be unpredictable. To someone who hasn’t tried it, it may look as if you have the drop on him. You don’t, quite.
As a final experiment, have him stand ten feet away from you with his back turned to you and his hands in front of him. Assume that he is a suspect fleeing from you down an alley, and finds that it is a dead end. Have him hold two different objects in his hands — say, his wallet and his cell phone. The cell phone, let us say, is a gun.
Point your “gun” at him. Say “Hands up!” if you want. His job is to turn quickly toward you, with either the wallet or the cell phone in his gun hand. If he has chosen to turn with the “gun,” he should shoot you as quickly as he can.
Your job is to decide whether he is holding the “gun” or the wallet, and either shoot, or refrain from shooting, as appropriate. Have a body bag ready, because, if he turns with the gun, you won’t have a prayer.
If you want, try it with a Day-glo pink plastic banana and a realistic plastic gun. For that matter, have him turn either holding the cell phone or holding nothing. The results will be the same. The arithmetic of reaction time says that you cannot look at an object, decide what it is, and take action before he shoots. The one who acts first not only has the advantage of reaction time, which in itself would be enough to shoot first, but also the time needed for you to decide what to do. Not even close.
Think about trying it in a dark alley, after sprinting a hundred yards, chasing a criminal who as a matter of statistical fact is quite likely to be armed, but may not be. Very likely, someone will die: You, because it was a gun and you waited to see, or him, because, gun or not, you didn’t wait.