I think that that BMW and the manufacture of the knife needs to be held accountable. Who sold him (or purchased for him) that car? Who sold him that knife? Did they do a background check? Was there a 10 day waiting period?
His parents bankrolled the guns and the car. If his parents had insisted he pay his own way through life he would have been riding a bicycle and using plastic knives pilfered from McDonalds. Guns would have been outside his budget.