The only thing about the Second Amendment that is certain is that it was intended to assure the survival of the fledgling nation that drafted it. That nation is now a superpower with no real need of a standing militia to support it's military might, but the possibility that it might not always be a superpower, or still be a constitutional superpower, still argues the inherent and dire wisdom of the Second.
To trust the common man with the right to keep and bear arms is emblematic of the kind of nation we intended ourselves to be, a democracy in which governance is in the purview of that same man. If we can trust ourselves to govern ourselves then we can trust ourselves to own firearms. Reciprocally, it would seem that if we can no longer trust each other to own firearms, we can also no longer trust each other to participate in governance. We want to have a country in which guns and governance can coexist. If we don't, then we will no longer have the country intended by the framers of our constitution.
A double-edged sword is fabled for the property that it "cuts both ways." It is a more dangerous sword to face and a more dangerous sword to wield. The Second Amendment is that kind of weapon. To wield it makes us stronger and imperils us at the same time. The death toll of our untutored, our youth, our mentally unfit and our innocents has grown in proportion to the lethality of the weapons it guarantees us a right to own. Our double-edged sword is capable of cutting both ways more lethally on foe and friend alike than ever before. The fact is that for us to uphold the Second there is a cost in casualties and fatalities that has, more recently, vastly exceeded the utility of the Second.
If we are unable to find a way to handle this weapon, the Second, without more imperiling ourselves than our foes on any given day, then we may not yet be able to govern ourselves the way our constitution proposed that we could, and may so not yet deserve this or the other liberties appurtenant to that constitution. We are a country in which, having wrested rights from an autocracy, now still furiously labor to apportion some finite amount of liberty among ourselves.
Liberty in not finite. Neither is responsibility.