These were entities ALL states had, as mandated under the Articles.
"every State shall always keep up a well-regulated and disciplined militia, sufficiently armed and accoutered"
And THESE entities were now given VERY specific duties in the new constitution for securing all our liberties.
"Congress shall...provide for calling forth the Militia to execute the Laws of the Union,
suppress Insurrections and repel Invasions;"
"The President shall be Commander in Chief of the Army and Navy of the United
States, and of the Militia of the several States, when called into the actual
Service "
Disbanning them would be an extreme form of tyranny, even without the 2nd amendment to secure them. Now, mention the ability of the congress to destroy them by DISarming them due to their new powers of arming and then you have the primary purpose for the 2nd.
Well regulated militias were important, not just in slave sates, but in free states too - for dealing with insurrections like in MA and PA, enforcing laws, serving as defence from invasion; and VITAL in the federal Constitutionally -mandated roles, required for intial common defence, repealing invasions, etc. etc. And especially necessary for removing the pretext for large standing armies. As the amendment debates in congress clearly show.
Of course putting down slave rebellions was a part of it - for slave states. They had particular interest in maintaing some control over them in light "of their situation", as the Virgina ratifying debates clearly show. They didn't want them to be rendered useless or destroyed by over-use all across the continent, harsh martial law treatment, etc. However, they also realized effective militias, meaning well-regulated, with uniform organization and training via federal guidleines, was clearly an advantage overall.
The 2nd is not all about slavery. Not by a long shot. Just like 'individual private use of arms', slavery was not even mentioned in the congressional debates.
Clearer now?
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