Okay, I'll grant you that there are some absolutists lurking amongst those of us who believe that the 2nd Amendment is perfectly clear and means what it says. You may be cheered to know that there aren't that many who are that absolutist.
But at the risk of being labeled a gun-rights nut, let me explain the quandary.
The problem with drawing a line -- any line -- is that it allows a corrupt government to define you as being over that line, and take away your God-given rights. They don't like you? All they have to do is arrest and convict you of something, or declare you insane (and that's not very hard, if they want to do it).
Be very careful in defining that line, because you could find yourself on the wrong side of it, all too easily, in the precise circumstance when you need to be on the right side of it.
That said, I don't think that someone who is certifiably insane or a convicted serial killer should have unrestricted access to fully-automatic weapons, either. But in a sane world, those people would not be out loose on the street, either. The crazy would be in a rubber room for life, and the serial killer would be pushing up daisies; problem solved.
For example, would the populace stand for a city or State limiting our freedom of speech the way they limit our freedom to bear arms?
I'd like to see the reaction if the State of Texas sued NBC, CBS, and ABC because it thought the content of their newscasts were dangerous.
A good point. Prior to the adopted wording, the 2nd amendment read
A well regulated militia, composed of the body of the people, being the best security of a free state, the right of the people to keep and bear arms shall not be infringed; but no person religiously scrupulous shall be compelled to bear arms
The italicized portion was dropped out of concern that a state, by designating a group or religion pacifist, could arbitrarily attempt to deny the individual right to bear arms.
A shame they didn't leave the composed of the body of the people in.