The assertion that opposition to the President is racist has been repeated so often, in so many forms, that it has become something of a joke on the Right. The older and more dangerous claim is that traditional American culture is inherently racist, in need of elimination (or at least 'fundamental transformation') because of the evil at its root.
So it must be no surprise to see this story asserting that the whole point of the Second Amendment was slave control. The intent of the argument is to suggest that the Second Amendment has evil bred in its bones, the sort of thing a decent society would thrust out.
The problem is, of course, that militias were desired and used for many reasons other than slave control -- indeed, non-slave states used them too. They were used to guard and respond against insurgencies, to repel and deter raids by Native Americans (a purpose also currently thought illegitimate by many, but highly understandable if you remember the women and children the militiamen hoped to protect), and for police purposes in an era when formal police forces were rare or expensive. They were used here in Georgia to deter Spanish incursions (as well as to make incursions on Spanish Florida). They were used as organizing institutions for the community, helping it to cohere and build a common culture from immigrants on a frontier. They were used as the backbone of Colonial resistance to British authority, and their officers provided the Colonial army with much of its early leadership.
In other words, it is very far from true that the Second Amendment owes its existence to slavery. Of course there is also a problem with reducing the Second Amendment to the militia: there is an individual right protected, as well as the state's interest in having a militia. Even taking that as an assumption, though, the argument is weak.