It doesn't mean "bad" as in "evil." It means "bad" as in the bread in the back of your cupboard that you forgot about and that's been there for 6 months and that if it weren't for the packaging you wouldn't even know what it was. It means "bad" as in the refrigerated, once-used jar of spaghetti sauce with the lovely whitish-green efflorescence growing on the inside. That kind of "bad."
And by "myth" I mean "story that purveys a commonly understood, accepted guiding principle or ideology." Which is by the way, the true meaning of the word. But like so many ideas in this world, the true meaning has gone almost completely "bad." Maybe with this one we can skim off the mold and retrieve it, just this one time. You ok with that?
So (as every explanation in this age must begin) back in 1787 when the USA was codified, there was a good myth. The good myth was that a free people must defend its own freedom; standing armies of professional soldiers--such as the one maintained by the British--were dangerous to liberty. A free people could not outsource the responsibility of the common defense to professionals. To do so would vitiate the virtue of the people. A free people must vote, a free people must serve on juries, and a free people must provide for the common defense. It was not a matter of choice. It was a matter of duty.
So strong was this belief at the time that every state had a statute requiring--requiring--compulsory militia service of all adult citizens. Not only was this a hedge against a standing army, but it was prima facie the ideal for a free people: "A well-regulated militia, composed of the body of the people, trained to arms, is the proper, natural, and safe defence of a free State."
These words were composed for the Virginia Declaration of Rights in 1776 by George Mason, who also went on to lead the fight for an amended U.S. Constitution. He was of the opinion that the draft being considered for approval by the people was soft on standing armies. At his insistence, by way of James Madison's leadership in the first Congress, the new Constitution included as its 2nd Amendment verbiage designed to keep the nation ever-reliant on a citizen militia serving as a matter of compulsory duty.
It didn't work out that way. The loaf got stuck way back in the cupboard. Militia service as a matter of duty was just too hard and too unpopular. You had to fine people that didn't turn out for the annual muster. Legislatures allowed an end-around by which people with money could furnish a paid substitute. Plus, there always seemed to be a few gung-ho individuals who were willing to volunteer to do the job. So naturally--eh, naturally--voluntarism replaced duty.
Thus it was by the 1830's, when Tocqueville was taking a deep look at democracy in America, militia service as a matter of duty was already moribund. No, worse than that: it had already rotted away to the point that Tocqueville got the facts wrong, saying that voluntary service was the only kind there had ever been, and that compulsory service was antithetical to the democratic ideal. Whew! Something stinks in here! And don't blame Tocqueville's French cheese!
Simply put, the spirit of '76 militia ideal says that all citizens, as a matter of duty, serve as trained, drilled citizen-soldiers in order to prevent the formation of a standing army. That is what the 2nd Amendment was meant to guarantee. By the 1830's the ideal had already gone bad. The 2nd Amendment was defunct. The institution it was designed to support had been allowed to disappear.
Today? Poor George Mason is doing the tilt-a-whirl in the grave. The USA worships its huge standing army. People have this idea that the 2nd Amendment has to do with personal, individual self-protection rather than about obligatory civic duty. And forget about "well-regulated." Professional policemen charged with civic defense are killing citizens with alarming regularity, while "concealed carry" licenses can be obtained without ever firing a weapon.
What to do when a good myth goes bad?
At least, in the first place, we can understand that it has, so that we can stop following the guidance of people who would stuff moldy loaf into our faces.
We can also stop to think that, if we want to live free in an ordered democracy, we're not supposed to have a choice with some things, like voting and jury service and, yes, militia duty. Those things are supposed to be a citizen's obligations.
Beyond that, I don't know, except to say that Jesus is coming soon. Or was.