Let me put it into a clearer perspective. Weapons ARE invented. Since the dawn of mankind they have been used to force people to do things they would not normally do. If all weapons disappeared I can assure you that I would invent some kind of a weapon and use it to subjugate everyone else. Eventually I would have to fight with other a$$holes like me.
Consider the other a$$hole owns a private plane. Is that your excuse to station stingers in your household? And if he is a farmer and own a bulldozer, do you field a Milan in your garden?
It's just human nature. There is no eliminating the "weapon" factor. However, if YOU had a weapon as well, I would not attack you because I might lose. I would build a weapon better than yours so I could attack you if I wanted to. But then you see me building a better weapon and make an even better weapon.........blah blah blah
I can'T see this weapon being any better. Only being more barbaric. certainly, there are cultural influences as well. People can learn not fo love violence and weapons. But my impression is, in american culture, medias and real life, both is cult - more cult than in any other western country.
As far as your argument that weapons are to easy to use goes, so are cars. Millions of people worldwide are killed by automobiles and no one cares because no one wants to see them get banned since they rely on them. But for gun-haters it is easy to hate guns because the vast majority of them have never used one. Cars kill more people than guns, why don't we ban them?
I don't know about you, but usually people board cars with the intention not to hit something. when you draw a weapon, that intention reverses to it's opposite. I knew that somebody would make the car compairson - it always pops up in discussions like this.
Always, really. Telling by years of multiple-threads-long experience.
Clubs are also easy to use; swing at target's head with maximum force. Care to ban blunt objects?
If they carry a short chain with an iron ball with stings at the top, yes.
I will make no argument against the fact that my gun (Walther P38) is for killing people who attack me or another person. Sometimes I shoot cans with it for fun.
And sometimes you stab cushions for fun as well with this gas-knife?
The only thing that would stop me from using a grenade or a machine gun (if they were legal) is the potential for collateral damage. A crimninal would have no such objections, which is why I need a weapon to defend myslef.
Yes, maybe. But you do not need an army, nor military equipment for that. Three years ago I was attacked on open street by a maniac, with a knife. Former training (19 years) and old reflexes helped me to block that stab and suffer only a cut on the hip while taking him out immediately. and I had no weapon at all. The point is, that training went hand in hand with education and training of my character. It changed me. Going into a shop and buying a tool of death - every maniac can do thatcan do that. He wopuld be better served with such a training. consiedet the times of Mus
If anything I would argue only for more training in the use of firearms so that we do not have as many accidental shootings.
Okay, my father is sports shooter himself, air pistol precision shooting, and cal 38. I shot some rounds, but it is not my thing, mine was archery, and swords, and martial arts, and I was good at all. He learned theory for half a year before getting a license. and he does not need dumdum bullets, neither for sports, nor for self defense.
so, why is such a knife needed? and why does it need to hide behind ridiculous claims like being the ideal weapon for self defense against huge land predators?
How about we have a contest where I am armed with an illegal gun and am intent on killing you and stealing all your stuff and you are armed with anti-gun rhetoric.
I bet I can kill you and steal your stuff well before the police ever arrive to protect you. 
Eventually if the situation favours your options. Else i would not be so sure of that.
As noble and theoretically noble the ban on any kind of weapon may be, reality says that you will regret not having said weapons.
Which does not answer the question: why such a barbaric knife? It even does not free you from the risk to close in to hand-combat range, which always is a risk. you can't even throw the thing, since oyu need to push a button to fire the gas. There is as much need for it than there is for cop-killer ammunition (those flechette rounds that penetrate protection jackets).