Thad wrote:And then there's demographics. As you said, shooters are likelier to be right-wing -- and I think that's down to the profile. Shooters are likelier to be white and male; white males are likelier to be Republicans.
The one thing I do want to say is that we're used to this being the case because it's been true for about the last 40-odd years - more than our lifetimes anyway. But prior to that, the US had a number of armed left-wing groups. Some unfairly tarred as terrorists (Black Panthers) and some with very explicitly violent intentions (Weather Underground).
If you go back further you see other groups like early unions willing to fight back against owners (in fairness this is after many owners hired armed goons), abolitionists, and even Temperance groups! (well, I'm not sure if Temperance groups count as left wing or right wing, but they were all or heavily women at time when groups of armed women was nearly unthinkable)
So the history is there. Sadly the US has a long history of people being or feeling disempowered and so resorting to "the cartridge box".