Any police officer that can't tell the difference between a pen and a knife from a few feet away doesn't belong on the force, simply on grounds of low IQ alone.
However, I would note that when the officer is not in possession of all the facts, that is the absolute worst time for him to be whipping out his sidearm and blasting away. Theoretically, use of the service pistol is supposed to be an absolute last resort, the kind of thing you do only once you have positive ascertained that a credible threat actually exists - not when you just see an object in somebody's hand that you haven't identified as a weapon - especially when the assailant is a one-armed man in a wheelchair. Don't they teach tactical sense at cop school anymore?