Agreed, the whole Antifa movement is IMO just crawling with the same anarchists who take over any protest for the past twenty years and make it into their own “Let’s smash some windows and burn something” event. Very unhelpful, but not that particular to the anti-Nazi protests either. But it is complicated by the fact that punching Nazis is, in theory, a very attractive proposition to most of the country, so they finally have a message that resonates a bit. There may be some sort of core legitimate Antifa movement, but at least from here they look outnumbered by anarchist jerkwads, and the non-organized aspect of it makes it pretty impossible to distinguish one from the other.
It also makes them kinda hard to reject by name, since they’ll just show up to smash windows and start fights under some other name (Were they called Antifa at the WTO? Nope). So IMO all the Left can do is reject the violence, and any individuals who get caught.