It is a coincidence in that the shooters were not explicitly organized, but with trends under Trump I firmly repeat that the ideological and political component of the violence is an important element in the recent timeframe. Gun availability and gun culture is one thing (and don't forget
toxic masculinity), but there is an additional layer in the echoing of 'lone wolves who all happen to believe what the President believes and want the same things.' That's the gap between societal ill and sectarian violence.
Edit: In a pleasing turn if you hate all subtlety,
this was one of the photos the El Paso shooter Liked with his Facebook profile. One of the contemporary memes floating around is that the universe needs to fire its writing team, the implicit metaphysics of which you would probably remark on.
Bookmarks