Because of the way you leftist nutjobs talk, is all we need to reinforce the need the framers saw in the 2A
The founders crapped in chamber-pots and saw bleeding as a valid medical treatment. I think we can safely dismiss them on authorities on anything.
Now, yeah, it might have made sense in 1787 to have an armed citizen's militia when you had slow communications and transport, no professional police forces and no standing army. THat might have even been a good idea back then.
today- it really makes no sense to let people like Mercer, Loughner, Holmes, Lanza, Cho*, etc. have access to guns despite everyone in their lives knowing they are mentally unstable.
I don't live in such fear of my government that I am willing to live in fear of these kinds of assholes. That's like filling your yard with poisonous snakes because you are afraid of burglars.
"Pa, a snake just bit little Sally!"
"Well, at least we kept the burglars out!"
(*- the problem with a list of mass shooters is that you have to make editorial judgements of who is bad enough to get on the list.)