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.
Yes you can kill people in lots of ways, but is that what happen?. There were over 11,000 gun deaths in the US in 2013. There were about 300 vehicular homicides in the same period. (link). People don't use cars to murder people nearly as often. It is interesting that there are more requirements for car ownership than gun ownership! It is a complicated issue. Here's a good article on the science comparing guns and crime in various countries. Some countries do well (Scandanavia for example). Some do very poorly (like Russia). It's a combination of laws and socioeconomics. In Norway, to own a gun you must document a need for a gun. Usually this is hunting or sports. Almost certainly not for self-defense unless you are a trained guard. You must obtain a license, which requires at least 9 hours training and a written test, at least 6 month membership and active participation in a gun club and afterwards a letter of recommendation from the club president. There's lots of other laws and restrictions, but I find this gun club requirement part of the law fascinating. It guarantees that a gun owner has been a part of a social gun group and understands what is considered proper behavior by peers and society. Compare to the US where Joe the Disgruntled can wake up one day and decide to go to a gun show and come home with a shiny new problem-solver. As far as energy density goes as you say, this is interesting also! What is clear is that some weapons are universally legal (fists, ballpoint pens) and some are not (tanks, missiles). In between is an entire spectrum of weaponry, and a line must be drawn somewhere. A lot of debate is on where that line should be. I don't buy the reasoning that moving the line towards higher energy density weaponry makes anyone safer. What about when the day comes when everyone has the power to kill anyone else just by thinking about it?
I've been wondering about this for months. There will come a time in the not-so-distant future where an extremely high percentage of the population will have the ability to kill most if not all of the rest of population. I can't see any way to deal with this except the following. Theoretically, once we get self-sustaining off-planet populations the existential risk is reduced by a massive amount. I see this weird pseudo-utopia as a possibility where families live miles and miles apart, on self-sufficient homesteads, kids and most adults telecommute to work or school, and travel only for social, or entertainment purposes. Have some communal areas, and cities will still probably exist in some form, but keep the vast majority of the population distributed, and hopefully, in a happy fashion.What about when the day comes when everyone has the power to kill anyone else just by thinking about it?