Never did I say ANTIFA was an organization, but a riot partially spurred by ANTIFA means a group, not a single person. If you or someone else shows support of the same group, it is to be expected that you align with their beliefs. It's like wearing a Trump hat into Charlottesville.
No. I have zero clue why you'd assume that. My friend was a regular PolySci student. They were walking to school, and a riot was happening on the street outside campus. There was a lot of disregard for who things were happening to, it seemed like people touting ANTIFA and such just wanted an excuse to be violent. The other group involved was the same. In the end it accomplished nothing for anyone.
The problem with that thinking is when people aren't opposing any brutal resistance. In the case of the protests/riots I saw, there was an alt-right person coming to speak in the area. People didn't seem to like that very much (for obvious reasons). This prompted both alt-right people and ANTIFA to scrum along the streets of downtown. It wasn't fun for anyone. There were no good guys, just idiots bludgeoning other idiots and also hurting Innocents.
So basically, it doesn't matter if ANTIFA is an organization or not. It doesn't perform the basic tasks it is supposed to perform correctly (violent resistance against oppression). It causes more damage than it's worth. In my experience, the people claiming ANTIFA are out for violence and don't much care who they are being violent towards regarding oppression. In the case of the protest in my example, they were actually a functional oppressor.