You’re using a very specialized definition of militia that does not fit the general usage, nor, indeed, the usage during the founder’s times. Militias were organized, they trained together, they knew each other. They were not vigilante gangs spontaneously forming on the spot.

The 2nd amendment was written with, in part, a mind to armed resistance against oppressive government. That is one of the major reasons why it was in there, both from states-rights aficionados who feared a federal takeover of the states, and from those who simply thought that, this who revolution and starting a new country thing was kind of new, it might be necessary.

Some 2nd amendment supporters these days still support that rationale for the 2nd amendment. The majority of them, being sensible and rational folk, don’t. Nonetheless, that is part of the original intention of the 2nd amendment, along with the support for the militias.

The 2nd amendment is also written for the protection of the state, against outside aggressors or insurrectionists. Most of the founders did not like the idea of a standing army.

Nowhere in the 2nd amendment is it written that arms are permissible in self-defense, because it barely needs to be said. The carrying and using of weapons used to be the privilege of the aristocracy, but Revolution changes things, and going about armed was the right of every citizen— except, of course, for black people. Gun control was already in place even in pre-Revolutionary America, preventing free blacks from owning guns, for the very honest reason of preventing insurrection by blacks. By laws like this, both before and after the Revolution, we can see that using arms in self-defense was not at all a contentious issue for the founders.

So, we can see that the 2nd amendment was crafted against a certain set of historical situations, most of which are now totally archaic and no longer relevant— much like the 3rd amendment which deals with a situation that simply doesn’t arise.

I would really rather conversation about gun control not fall back to appeals to the 2nd amendment, but to the basic rights of people to be safe and secure in their persons and use reasonable means to defend themselves. Things have changed since the revolution. Guns have changed. Society has changed. The needs of the state for a militia have changed, and the needs of a citizenry to have a militia have changed.