As I think you've noted previously, there's a lot of guns in the USA.
Luigi was one of four recent, last 12 months, people in the news whom I would count as… well, I'm not sure "left" is the right word, but more generically not-the-Right Americans with guns who I would categorise as an "extremist".
(The other three being Matthew Alan Livelsberger (even if he only shot himself, given he also blew up the car he was in), Ryan Routh (golf course), and Thomas Matthew Crooks (ear)).
Like you, I find it remarkable how the US has managed to combine the 2nd amendment and lots of school shootings with relatively little political violence. Like, why are their politicians more sacrosanct than their own kids? Why is the 2nd amendment sacrosanct even to politicians who get the dangerous end pointed at them?