"trapped one of the officers with his wheelchair...attempted to stab the officer with an object..."
It wasn't the trapped officer who shot, it was their partner, who had apparently just entered the room to find their partner trapped in a corner by the guy in the wheelchair, who had something in his hand, and was acting as though he intended to stab the other officer with it. He acted to prevent his partner from getting stabbed (or so he thought). Yes the outcome sucks, but the options for preventing a stabbing when the assailant has closed to within reach of their intended victim are limited, to say the least.
How many of you have heard of the 21 ft rule?