Regarding “far-Left” vs. “far-RIght” extremism... right now in the U.S. and in most of the Western world, far-Right extremism is a far bigger, more immediate, and far far more violent threat than far-Left. Far-Right extremism is almost singularly the immediate cause of backsliding libral democracies.
The underlying causes of the rise of far-Right nationalism (ethno-nationalism, religious nationalism) are complicated and intertwined and I would not let anyone completely off the hook; though, even setting aside Russia and trolls and bots, I believe the insular and simplistic worlds that social media has created are the single biggest cause. But by far the greatest current threats to libral democracy in the present — both in the U.S. and in other heretofore libral democracies — come from the Right.
This (1) does not mean we should leave the illiberal aspects of the far Left alone to fester and grow. The far Left has gotten bigger, more powerful, and more violent in recent years. And (2) it does not mean that far Left extremism doesn’t have the capacity to be just as illiberal, violent, and destructive of the human spirit. While I cannot think of a singular act as evil as the Holocaust — perpetrated by people we generally ascribe to the Right, though it is a little more complicated than that — between China and the USSR, the far Left is at least as responsible for overall global suffering during the 20th century as the far Right. Even now, China is putting millions of Uighur Muslims is concertration camps and re-educating them... and harvesting their organs.
So, to reiterate, in the U.S. and in the Western world, generally, in 2019, far Right extremism is by far the biggest political threat we face. In fact, in the U.S. it is currently the biggest terrorist threat we face. (And of course the ideologies of these assholes are being propped up by our current President.) But it does not follow that far Right extremism is intrinsically worse or more dangerous; or that far Left extremism should not also be taken seriously as a threat to libral democracy and addressed accordingly.