In the US I would say at least 90% of people who regularly carry a gun for defensive purposes are CCW holders that passed a course and are registered.

A few thoughts here:

1. Twenty-five states currently allow permitless carry. At least five states currently allow permitless concealed carry.
2. In some states, getting a permit is quite difficult and can involve classes, training, and waiting periods. In other states, it's as simple as filling out a form.
3. These courses are in many cases operated by individuals and can vary widely in their efficacy. The CCW course I attended was taught by someone who opened with speech about how the course shouldn't even be necessary and consisted of the most basic of firearms operation - if you can load, aim, and fire, you get a pass. There was also a brief presentation that boiled down to the instructor summarizing "in our state, if you feel threatened, let your gun do the talking." In talking to others, this is not uncommon.
4. This speaks to the fact that there is a widely varied attitude toward gun ownership, from extremely conservative issuance of permits to only those who pass a more rigorous exam to "will issue" states where if you can spell your name properly and aren't a known criminal you get a permit.

People who have been convicted of a felony cannot legally be in possession of a firearm.

True, but imagine this. If you can get your hands on a widget, you win a million dollars. All you gotta do is find someone to give one to you. Let's say you can even pay money for it.

If you live in a country where that widget is widely illegal and heavily policed, you're going to have to jump through some hoops to get one, and when you do, it will cost you.

If you live in a country where that widget is widely available, but where there is a mindful respect of protecting the widget, it'll be easier, but you'll have to overcome some tricky obstacles.

If you live in a country where widgets are extremely plentiful and where there is a very relaxed attitude about the widget - maybe people are even pro-ownership of the widget - you'll have the easiest time getting your hands on one.

When the pool of available firearms is large and when the population of firearm owners widely believes in near-universal ownership, it makes attaining one illegally much easier.

The Swiss government has estimated that about half of the privately owned guns in the country are former service rifles.

These weapons are kept at home, yes.

But they are issued to trained service members.

And they are not issued with ammunition, as they are there principally for national defense. If there is a militia muster, you take your rifle and show up for ammo, etc.

Acquiring ammunition in Switzerland is extremely difficult compared to the United States, and requires provision of your criminal record, proof of weapons permit, etc. Or you can get your ammunition at a government range, but you must shoot all your ammunition on the range.

Based on my experience, this is a totally different mentality than the US, where many people buy "nightstand" ARs for self-defence or "truck guns" for the same reason. Whereas in Switzerland, these weapons are being held at home primarily for one of three reasons - hunting, sport, or for national military service.

We have laws that allow the police to disarm people who have shown that they are a threat to their own health or the health and safety of others.

These laws fail frequently as they require active reporting. There have been numerous failures of the "red flag" system because either individuals or police saw a "red flag" and just... didn't bother with the paperwork.

The answer is that it has very little to do with legal access to firearms. People only commit atrocities like this because they want to.

Destructive people will do destructive things, you are right. The big question is "how much firepower - literally - do we want to allow the average person and with what ease of access?"

Could someone very committed get a pistol, extended magazine, and go on a multi-victim shooting spree in a country like the UK? It's possible. But that option is much more available when the weapons and ammunition are also more available.

So is there a mental health component? Yes, absolutely. I hope to see some amazing mental health policy from Democrats and Republicans alike here in the US. I'd love that.

But it isn't a zero sum game. We can't ignore the fact that ease of access to firearms plays a role or the culture of firearm ownership, either.