Man charged with threatening to kill U.S. Rep. Katko over 'net neutrality'

That doesn't properly explain the freak-out from the extreme left ~45 years ago, which resulted in dramatically more violence and terrorism than the recent extreme right freak-out. There was no loneliness epidemic at the time among that group.

Which is to say, there are plenty of other plausible explanations to consider when a large group of people start behaving in what is an abnormal manner in contrast to the prevailing culture.

Let's see. There was a leftist that shot up a congressional ball game. There's this guy.

Have there been any Republicans that threatened legislators?

Eliminationist, xenophobic, hate-filled rhetoric has been spewing from mainstream right-wing media and leaders for a very long time now. If you add that to the extreme right's roots in violent extremist groups like the KKK and the Nazis, it's really no surprise when they start acting out the violent fantasies they've been egged in to by their leaders and spokespeople for decades.

Mainstream left-wing media and leaders have not had corresponding calls to violence, though there are violent extremist groups on the left too, and a bloody history from Communist revolutions, Weather Underground, Red Army Faction, etc.

What's missing from the leaders of both sides are strong commitments to non-violence, as there were in the Civil Rights struggle in the US and Indian struggle for independence back in the 20th Century. Only something like that has a hope of defusing the ever-growing political powder kegs in the US and around the world.