Non lethal? In this situation? That's your daughter who is getting ready to take a long bladed knife to the chest, kidney, throat slit perhaps not once but multiple times. A split fucking second. That's all it would take and your daughter is laying on the ground bleeding out and dying. And you would like this officer to just "intervene"? Shoot an extremity? What if that didn't stop the attacker? What then? Shoot another extremity? I have the feeling you would be thinking very differently if this was your kid.
The people who do shit like this bring it ALL on themselves. They bear total responsibility. Not the Police Officer who was called, came and did what he had to do to saved that young girls life.