It does constitute an opinion, but an incorrect one. Taking over a police station and a city hall in Seattle, for example, is pretty impressive - and it strikes violence at the heart of perceived violence.
No, Antifa is not a club or an organization. There is no membership. And there is no bouncer at the protests to make sure everyone is on the same page or of the same opinion - it is flat out inevitable that some people will be of poor opinion and behavior.
The Right's framing of the relatively small amount of violence is indicative of a police state. The reaction by police unions in the face of calls for punishing abusive cops is indicative of the violent arm of the state protecting its own absolute authority over the people.