That is a popular assertion. But it soesn't hold up to either historic scrutiny or an examination of our laws.
Where in the law was the 2nd used as a defense against insurrection? The Confederates didn't use it, the members of the Whiskey Rebellion didn't use it to excuse their actions. We've had several armed actions against the federal government and no one said, no harm no foul they were practicing their second amendment rights.
If you are scrutinizing it with the mindset that the 2nd Amendment is a justification or an excuse for armed insurrection, you won't find it. Because its not. And, its not even close.
The 2nd Amendment grants nothing to the individual citizen. It is a check on the power of government. period.
People get too focused on the reasons (real or imagined) behind the 2nd Amendment, and not on the article itself.
None of the examples mentioned ever did try to use the 2nd Amendment as any kind of defense for their actions. Nor were they accused of illegal ownership and carry of arms (keep and bear). They were charged (and fought) because of their illegal
use of arms. There is a difference, though many do not, or will not recognize that difference.
This is the mindset of the anti-gun bigot. They believe that if you have a gun, you
will use it to illegally shoot someone. They cannot seem to conceive of anything else, and therefore believe guns are evil, need to be restricted, and banned whenever possible.
As for " private armies to put down slave revolts", do remember that while they can be used for that, at the time people were also concerned about attacks from "wild Indians" and more than a bit worried about the troops of foreign Monarchs. Britain, France, and Spain all had troops in North America at the time, and we had been fighting some of them for decades.
And, we fought some of them again, after the Constitution was ratified! also remember that at the time, there weren't even any police in our new country. Organized police forces didn't exist then, and didn't arrive on our social scene until several decades later!
The reason we have arms is because it is our natural right. The 2nd Amendment formally tells government to keep their hands off that right.
The fact that it has been less than 100% successful in doing so over the recent century is a different matter.