Sucks. But I don't know why right extremist views are a threat to society specifically.
Because their target is society as we know it.
I don't know what that means but left extremist views are just a source damaging to society.
Sure, all extremist views are damaging to society. But the destruction of society is merely a byproduct of extreme leftism, whereas the destruction of society is a main goal of radical right extremists. Plus the extreme left doesn't tend to form into groups and arm themselves and carry out plans to destroy society like the extreme right does.
Both of them are bad, and really neither of them has any chance of succeeding in taking over society. But the extreme rightists are far more violent and willing to kill, as evidenced by the white supremacist in Charlottesville.
Antifa has displayed that the extreme left can be just as violent. I don't see either being worse than the other. Two sides of the same coin, methinks.