Background on the 2nd Amendment

Let your mind run back in time, to 1789, the year the constitution was adopted.  The United States possessed an enormous territory, stretching 2000 miles from Maine to Georgia and inland for a thousand miles.  It was thinly settled in those days.  France and England were the super powers of the late eighteen century and everyone understood that one or both of  them would want to expand their power by taking over parts of the brand new United States. Nearly every settled place had seen Indian raids, banditry, pirates, French, Spanish, and lastly Redcoats.  No way the infant federal government could protect  this huge vulnerable territory with regular army soldiers.  They lacked the money, the supplies, the roads, and the shipping, to get regular army troops into position to protect the civilians from all the potential attackers.
   The Americans had just finished the Revolutionary War, where American militia had driven Redcoat regulars into flight from Concord, slaughtered them en masse at Bunker Hill, forced "Gentleman Johnny" Burgoyne to surrender an entire British army, and served with distinction on hundreds of battlefields.  In those days everyone knew the militia was needed for, and adequate for, protection of American civilians, anywhere up and down the length and breadth of the land.  We would raise a small regular army, but for defense of the homeland, we would rely upon the militia.
   This was the thinking behind the clause "A well regulated Militia being necessary to the security of a free State...".   Militia was a bring your own gun (BYOG) thing.  In those days no state or federal government had the money to provide arms to the militia.  And it was also known in those days that plenty of land owners, patroons, and other colonial big shots were in favor of taking guns away from "the rabble" who might use them to cause trouble.  Hence "the right of the people to keep and bear Arms shall not be infringed." 
   And this worked for many years.  As late as 1940 Japanese admiral Yamamoto said "To invade the United States is impossible.  There would be a rifleman behind every blade of grass. "   America is no longer a shaky new found country clinging to the coast of a continental wilderness.  We are now the strongest country on earth with regular armed forces surely strong enough to defend the homeland. 
   But for all our modern improvements we still have reasons for citizens to want firearms.  For instance, I have black bears strolling up and down my driveway, especially during beechnut season.  Plenty of Americans live in far more dangerous places than I, and I don't see any reason to deny them firearms.   Plenty of robberies have been thwarted with the help of a gun in the cash drawer.  So have plenty of home invasions, muggings, and car jackings.
   The recent appalling  murders of school children and innocent spectators happens because we allow homicidal maniacs to run around loose until they commit an awful crime.  We used to have mental hospitals in which we confined those of unsound mind.  Unfortunately the civil rights movement of the 1960's forced their closure, and turned the inmates out into the street, where many of them still live.