If the people running this free speech gathering had any brains, they would have postponed the demonstration until people calmed down from the incidents that occurred in Charlottesville. The country as a whole, yes conservatives and liberals, are angry and still disgusted and the wound is very much still open. Any gathering that was even close in subject matter was going to be massively protested against and the chance for some violence would exist.
Yes the Boston Free Speech organizers denounced the Unite the Right organizers, stated vehemently that they had nothing to do with them and that their cause was completely different but they still should have postponed. If they had the same event in November, I doubt 40,000 counter protesters and protesters of radical groups like white supremists and the KKK would have shown up.