Antifa is basically "mob justice." They're violent and dangerous. They could easily hurt innocent bystanders because they get all worked up. This is why we have police.
Unfortunately majority of the Antifa movement is nothing but left wing socialists who assault people without being provoked. They justify these assaults by saying that "fascists don't deserve a platform". It doesn't occur to them that by physically attacking fascists just makes people sympathize with them along with giving them attention they may not have gotten on their own. Not to mention that the differences between socialism and fascism is basically marketing