The net result of advancing technology is ever-increasing energy densities. Whether that is chemical energy, electrical energy, or mechanical, is almost irrelevant, because we can convert between the three (with loss I'll admit) rather easily.
In my mind, a gun is just another method of force-multiplication. A guy with an AR-15 has a lot more applicable force than a guy, or several guys, with knives, spears, crushing implements, etc. But then, so does a guy with a bomb in his backpack. So does a guy with a tank of homemade mustard gas and a sprayer.
It's completely legal to own all sorts of things that can cause death to large numbers of other humans. Mass quantities of petroleum, gunpowder, etc.
Chemicals that, while innocuous in isolation, become deadly in mixture (Bleach + Ammonia = Mustard Gas)
I mean, for petes sakes, we drive CARS. A few tons of metal and plastic, fueled by an immensely dangerous and expensive petroleum distillate that doesn't even require open flame to ignite. A 16 year old is allowed behind the wheel by themselves, and could INSTANTANEOUSLY end the lives of a whole ton of people as quickly and easily as a dedicated 'holy warrior/freedom fighter/terrorist/insurgent/whatever word we're using now.'
Looking to the super far future (Or maybe not so far, depending on who you read) there will come a day where most of our species will have the ability (Not the motivation, but the ability) to end the lives of most of the rest of the species. These people exist now (The guys with keys to nukes and weaponized diseases mostly) and the number of them has never been higher in human history. And you know what? That number will ONLY GET BIGGER. All as a natural result of increasing energy densities.
In my opinion the question that we should be addressing, is how do you catch a person with violent anti-social tendencies before they can manifest that violence, rather than worrying about playing whack-a-mole with different modes of violence that can be circumvented legally anyway.