Are you looking for an answer to a question, or are you looking for a debate?
At any rate, reducing the utility of an item to what it's "lowest performance" should be to lower it's ability to harm for non-intended uses is asinine. Who sets the limits? Does a knife need to be razor sharp? I can cut a lot of things with a dull knife and some time. It would pose less danger to you if all knives I had access to were purposefully dull. To prevent me from procuring an overly sharp knife, make the material strong enough to cut foods, but brittle enough to not be one overly sharp. Knives, after all, we're made to stab, cut, and dissect a wide arrange of materials, flesh included. This specific design poses limitless danger to you, and needs to be considered when manufacturing these tools.
Guns are not majorly sold specifically to kill people, in the grand scheme of things. Hunting is probably the largest vector of volume gun sales in the US. How do you design a weapon that can be useful for hunting, but ineffective at killing a human? They all possess the innate ability to do so, but so does even the smallest pocket knife or kitchen knife.
I'm also a big gun control advocate, so I'm not defending anything I like. The failings of US gun control are squarely on the idea that everyone should possess a gun until they prove they shouldnt; it's reactive policy. Active gun control would limit who can possess a gun from the start to those that will only use it for "appropriate" reasons.