Creative Speculation

By 1787, when the constitution was written, the Revolutionary war had been over for a decade, and there were significant concerns, especially from the northern states that state militias might actually tear apart the nascent US Army, founded in 1775. However, the southern states had specifically created militias for the purpose of capturing escaped slaves and returning them to their owners, and their fear was that should there not be a formal statement to that effect that the North would force the South to give up slavery. This was the reason that this was given as the second amendment, rather than being one of the key fundamental rights given in the first (speech, press, freedom from search and seizure or unlawful billeting, and so forth) - there was a vague hope that as a separate amendment it could be overturned without overturning the others. As it turned out, even with this amendment the South would secede eighty years later.

The second amendment was a classic example of bad law - it was deliberately left vague and ambiguous, it served a very narrow parochial interest, and it was a compromise that ultimately would bring the country to its knees.