I almost never agree with you but you've called this one right. Advancing on a police officer, armed, and refusing orders to drop the weapon - he surely asked for it.
The police officer is not obligated to "subdue" such a clown in a way as to risk his life and the control of the situation. He subdued him in a very practical way.
My only question is "Why eleven shots?" Either it was overkill or the guy with the boxcutter was one tough, determined dude.
If the officer was justified to fire one shot, he was justified to fire eleven or twelve or whatever. Cops I know are trained to fire until the threat of deadly force is stopped. They are not taught to shoot to wound or subdue.