Antifa is part of the left, undoubtedly. Most of the positions they hold--outside of their propensity for violence--are in line with the rest of the left. The rest of their positions (like deplatforming right-wing speakers) are in line with the authoritarian left (as opposed to the libertarian left, which is outright gleeful to have idiots like Ben Shapiro, Steven Crowder, Ann Coulter, Milo Yiannopoulos, Richard Spencer, etc., speak because they just expose their hatred, bigotry, and stupidity whenever they do).
That doesn't mean we (or at least I) condone their actions. I'm in full agreement with Noam Chomsky--Antifa is the biggest gift the left can give the right, because the moment both sides accept violence, the more violent side will win, and that is undoubtedly the right.
Plus, resorting to violence (as the right is far more prone to do than the left) is an admission that you can't convince people of the correctness of your views any other way--which makes me despise Antifa all the more, because I'm convinced that the left is correct and does not need violence, because the left will win the war of ideas because it has superior ideas. The right needs violence because its ideas are atrocious and the only way it can impose them is to violently suppress opposing ideas.