A terrorist is anyone or group who's sole purpose is to use violence to push a political agenda. Thats exactly what Antifa is and doing. Your idea of terrorism probably defaults to groups like ISIS, but that isn't all terrorism. Then there is the distinction between a member of a group doing something bad, and an organization designed to do bad. Hence the reason the KKK and people like WBC are not labeled terrorists. Yet people like the Unabomber was. This being that the KKK doesn't enact violence for a political agenda, but usually take legal routes to follow their agenda (buying legal permits to protest etc). Even if you find their ideas harmful, its still protected by the first amendment. Antifa surrenders this right by attacking people, rioting, and burning down cities. While not all of them do (the useful idiots), the people controlling the operations want them to.
I don't support violence but given the way our police state is a terrorist institution (based on most definitions of the word), and comparing the extent and severity of violence and/or intimidation committed by the US police state vs that committed by Antifa, who do you think is the bigger terrorist threat?