That is an easy one: it is the alt-left. Proof? Once upon a time, a long time ago (1977) there was a neo-Nazi march in Skokie, IL. That town was comprised to a large extent not only by people of the Jewish faith, but many who had personally experienced the horrors of the Holocaust. Was there any violence on that occasion? To ask this is to answer it: there was not. Why not? Because in them thar far away days, the alt-left had not yet begun their pattern of intimidation of the sort suffered by Charles Murray, Ann Coulter, Milo Yiannopoulos and other conservatives and libertarians.
Actually, the reason there was no violence during the neo-Nazi march in Skokie is that there was no neo-Nazi march in Skokie.