Going out on a limb from which I will pull back depending on the actual range and scope of this kind of resistance (who gets targeted) and what one means by violence (standing the line, physical disruption, throwing fists or acts more serious) and .
There was this piece here by Dahlia Lithwick about the Antifas protecting life and limb at Charlottesville. And there is this political profile in the NYT, which led me to this Antifa webite here.
Here are 3 compelling, but quick and tentative arguments made by antifas that I culled from all three sources:  Fascists don’t want to talk, so talking is useless.  It is unthinkable to cede the streets to fascists.  It is urgent to hold the line, physically.
So these are a few quick questions. What would have happened if the antifas were not there as a physical barrier at Charlottesville protecting students Friday night or clergy the next day. Do Nazi marchers have a legitimate place in the public sphere?
Going only so far, the antifas deserve some cautious acknowledgment.
By definition, limits are there to be crossed. For that reason alone there is no reason to trust whatsoever that the antifas will stick to a sharp and narrow focus kept strictly on fascists (people marching as alt-right and/or under overt Nazi and white supremacist banners). All bets off if someone starts knifing and shooting.
antifa has been awfully disciplined in focusing on beating back fascism for decades now, not sure how to take yer sort of slippery-slope argument/caution, my only caveat is I don’t fear speech/demonstration/contagion and I don’t condone preemptive violence but self-defense is on the table.