Those with criminal intent, or the insane, or the politically-radicalized, or individuals who have "snapped" for one reason or another... Are notoriously not concerned with legalities.
I hear that a lot, but looking at the places where it is harder to get a gun legally, you'll find that most people tend NOT to:
A) want to escalate the risked punishment needlessly, nor
B) be criminal masterminds, when they do
The USA seems stuck into some fantasy alternate universe, where criminals are pretty much a cross between a Lex Luthor wannabe and Will E Coyote, and will do any inordinate amount of extra effort, for no other reason than to be even more of a Dick Dastardly type, so to speak.
You have to realize though, that most criminals aren't particularly smart about it, nor very industrious. If they were either, much less both, they'd be in management instead of crime. But really, does the guy robbing a 7/11 of a couple hundred bucks they had in the counter strike you as that kind of cunning and industrious mastermind?
The guy who snaps and shoots up the school, doesn't first spend years making contact with the Russian mafia smugglers, befriending them, and buying an AK-47 at inflated prices. In ALL cases I can remember, actually they used something they already had, or their parents had, or was trivial to get or improvise. (ANFO seems to be a recurring favourite.)
So basically I hope I can be unimpressed at the rethoric that if those didn't have trivial access to guns, somehow they'd be that determined and industrious as to get them anyway at all cost.
And, really, even in countries where access to weapons is easy (e.g., yes, the USA), you'll find that most people DON'T try to escalate their own punishment, barring idiotic laws that escalated it there already. If the option is between going to jail for a few months for whatever the guy was doing, or going to jail for life for shooting some cops, actually the vast majority DON'T pull a gun when a transaction turns out to be a sting. Real life accounts of DEA officers for example aren't anything like action movies.
Similarly, in countries where guns are hard to get legally, you'll find that actually most people DON'T upgrade a robbery with the added offense of illegally carrying a firearm.
I suppose the moral of the story is that caring about the law isn't a black and white thing. People aren't either 100% law abiding or 100% determined to break every law in the book.