Antifa?

 I'm pretty sure "Antifa" is just a placeholder for right wingers to aim attacks.  There are obviously lots of people people who are anti-fascist--I certainly am--but the number who are likely to do violent or destructive things is almost certainly minuscule. 

Here are some categories:

Peaceful Demonstrator:   These can be from any issue and with only a few very narrow exceptions, their activities are protected free speech.  I have participated in many, many peaceful demonstrations, such as the Women's marches, Anti-War demonstrations in the 1960s and early 70s and 2003 and have never once seen any sort of violence.

Rioter:  There has been some rioting in a tiny number of this years demonstrations.  I haven't actually seen any in person but I've seen a little on TV.  Rioting nearly always is counterproductive to any cause being peacefully demonstrated for.

Provocateur.  Nearly always, these are people who show up to undermine a peaceful protest.  Many strategies are used, such as starting fights, throwing rocks or other things, especially at police or windows.  They fall into two classes:

Opponents of the cause:  e.g. a right wing provocateur will infiltrate a left wing crowd and stir up trouble.  There were a lot of these in the protests this summer.

Nihilists.  Incorrectly called "anarchists", these are people who find pleasure in violence and mayhem.  They generally have little or no ideology.

The Cops:  Many cops actively oppose progressive causes and in the case of Black Lives Matter, they are representatives of the problematic group.  When they commit acts of violence against previously peaceful protesters, it is not the fault of the protesters that they get angry.   I think it's very significant that the violence in the BLM protests almost completely evaporated when the cops stopped enforcing curfew.

Looter:  like provocateurs, they are at the protest for a reason which is not aligned with the protest itself.  Some of them are there because they agree, but once there is a little broken glass, their main objective is to take advantage.

Anarchist:  There is a legitimate political strategy called anarchism, most conspicuously described by Kropotkin, but it has rarely gotten very far in practice. Kropotkin's idea was to antagonize the ruling class so they would crack down and make support for their uprising nearly universal.   Unfortunately for the strategy, there's an easy way to undermine this: punish the criminal and make a point of not cracking down.   The Portland/Eugene area seems to be a hotbed of people who call themselves anarchists.  They are not.  They are nihilists, who show up at riots wearing black with their identities obscured, and just there to make trouble.

White Supremacists:  This is by far the largest source of terrorism and organized violence in America.  Before Trump was elected, they were active but understood most people were against them, although the vast majority of mass bombings and shootings were done by them.  The vast majority of people arrested for doing violence in Black Lives Matters protests this summer were White Supremacists, trying to undermine the cause.

Religious Extremists:  This is #2 to the racists and they are often the same people, shooting up abortion clinics and so forth.

Antifa:  Appears to be a fictional group made up to be a stalking horse.   Every sane person is anti-fascists, although many Racists and Religious extremists are pro-fascist.  Not all, but enough to be a problem.   Nobody has been able to identify any actual antifa group although occasional a person arrested will admit that they are antifa.  I'll be interested to learn more about the person who was killed by the cops last week who apparently fits this category.

I live less than 10 blocks from virtually all of the protests that occurred in Seattle this summer.  Without the curfew announcements, the news reporting and the occasional news helicopter overhead, I would have had no idea it was happening.