We can reasonable assume that the attacker was no knife expert or she'd be dead. May just have time to pick up a TV and bash him in the head with it without shots fired. Saving her is saving her but if it can be done without shots fired, all the more prudent yes?
I could be wrong, but I think most states would consider hitting somebody over the head with a TV to be deadly force. You might as well just shoot him... same amount of legal hassle and you don't have to pay for a TV set.