Black-clad antifa members attack peaceful right-wing demonstrators in Berkeley
You do not see widespread condemnation of Antifas. You do not see them losing their jobs just for being observed at Antifa rally. You do not see them being banned from Ok Cupid just for being observed at an Antifa rally.
Why the glaring double standard between right-wing and left-wing extremism?
because "left-wing extremism" happens more or less in a vacuum - an isolated and independent group of individuals take action which is at best ignored by the mainstream "left" political body (and media, for what very little "leftist" media exists in the US) but is usually officially condemned, and there is no political clout to the actions of these individuals nor any notable public support.
right-wing extremism is openly embraced and encouraged by the elected body of representatives as well as the media, to the point where republicans show up on ranches to support domestic terrorists and pass laws to embolden or protect them, and there is often significant public support from the ilk that buys into this sort of crap.
so, when "left wing extremists" do something shitty, it's going to happen and then it's over and while it's a tragedy for the circumstance surrounding the incident there's no broader ramifications.
when "right wing extremists" start pulling shit, you're right around the corner from having their behavior codified into law.
thus, a more vigorous public reaction is required in order to set the precedent regarding the social zeitgeist, and/or to display public disapproval for those actions when none comes from official sources.