... the key difference between the one country where this generates a large number of killing (the US) and the other ones is clearly, without the shadow of a doubt, the free availability of guns.
This should be easy to acknowledge. I have a hard time understanding why we can't get past this obvious fact and debate based on this common view. The free ability of guns in the US is the enabler of these killings. Putting it the other way, removing the possibility to access gun freely would fix those killings to a large extend...
Obvious fact? To you, sure. To many others, especially here, it is just an opinion. Common view? Now you are departing from a "fact" to a "view," which is good, but common? Again, not here. Common outside the US? Probably.
The differences between countries can not be reduced
ab absurdum to just one "key" factor. Country cultures come as a package, i.e., as a totality of factors, and you can not cherry pick from one and transplant it to another. Many Americans would like to eat cheese and drink wine like French, and be healthy and skinny like French, but alas... By the same token, you can't just take guns out of the equation in America and still have... America.
As noted before, Chicago had guns banned for decades until recently, yet has had some of the highest gun violence rates in the country. As noted before, drugs are prohibited universally across the States, yet anyone who wants them can get them. If you ban guns universally, across the whole USA, not just certain cities, there will be a black market, just like for drugs. Another example on an international level: many war-torn countries are under arms embargo, yet they get theirs "freely" on the black market. American tried to ban alcohol universally, didn't work.
Just for the record, I neither have a gun, nor want one. I am all for a tighter control, definitely for a ban on high-power, military-style guns. However, I do realize it is only my personal choice and preference, my opinion, and realize there are other opinions too. I do not think that when each side sticks to their guns (pardon the pun), without listening to each other, without taking into account different viewpoints, we are not going to make much progress.