Noteworthy: “...the woman he was threatening was his own mother. He turned the knife on his mom when she refused to buy him a gun, according to a copy of the police report...”
I suspect the knife holder was a “definitely prohibited person” for gun ownership. Very likely mental illness. I would also guess he got the fillet knife at the store as well. (Outdoor department, both knives and guns.)
If this is the case, he is in for an extended stay at the Daisy Hill Puppy Farm.
You missed the most obvious answer to the issue. Walmart will henceforth cease to sell and and all sharp instruments and any and all firearms. Pretty easy solutions huh? :>}