I'm not quite sure what "mostly peaceful" means. I keep hearing this term in reference to the protests in Portland. Presumably it means something like "Most of the people involved in the protest aren't committing violence" or "Most of the time, there is no violence happening." It's certainly fair to get defensive if someone is trying to discredit a righteous protest movement by pointing to a few errant acts of violence and property destruction. I think that the vast majority of protesters aren't themselves committing acts of violence or vandalism.
Then again, I think it's easy to lampoon this concept of "mostly peaceful" if it's defined too broadly. My favorite example comes from Bret Weinstein and Heather Heying's Dark Horse Podcast. (I couldn't tell you which episode, unfortunately.) They point out that the procession of John F. Kennedy's presidential motorcade in Dallas, Texas was 99.9% non-violent. They also point to several wars in which the populations of the warring nations were mostly non-violent. (I believe they took some of these examples from another source, but I haven't been able to locate it.) The point is that you can't judge the violent or peaceful nature of something by the raw proportion of individuals behaving violently, or the fraction of time in which violence is happening. There are several criteria that may define a phenomenon as violent even if the "proportion" of violence (as a ratio of individuals or time or whatever) is relatively small.
- There is an implicit threat of violence far out of proportion to actual violence committed. (Such as protestors shouting "Get out of your homes and into the streets!" over a bull-horn at night in a residential neighborhood, while shining bright lights into the windows of darkened homes. Or, say, surrounding a restaurant patron at a sidewalk dining space and demanding them to pledge fealty to the cause.)
- Violence erupts at predictable times or under predictable conditions. (Weinstein gives the examples of the Portland protests turning into riots when night falls. The CHOP/CHAZ in Seattle might be another example of a "mostly peaceful" phenomenon with predictable violence at certain times of the day or night.)
- Supporters of the legitimate goals of the movement decline or refuse to distance themselves from acts of violence. (Obviously this is related to 1. A refusal to rule out violence means you implicitly align yourselves with violent enforcers.)
- Related to 3, there is a refusal to admit that violence has in fact happened.
- Also related to 3, activists who are looking for a fight find it very easy to attach themselves to and co-opt the otherwise legitimate protest movement. (I think some of the Antifa folks and left-anarchists have always been in the background looking for a street fight, and some of them have latched on to this as an excuse to start fights and destroy property.)
- The violence that does happen (or is merely threatened) has a disproportionate impact. It causes fear far out of proportion to the act itself. (Again, this is closely related to 1. "Setting an example" is a kind of implicit threat to engage in future violence, whether it actually happens or not.)
- Permissiveness toward minor threats or small acts of violence encourage or portend future acts of violence. (Think: failure to stop Hitler while he was merely saber-rattling, or when his hostile intentions became clear but he was still weak enough to be contained.)
- Advocacy of policy goals that will almost certainly lead to more violence. (I think suddenly abolishing the police and indiscriminately releasing felons, as some have advocated, qualify for this one.)
I could probably think of others, and I could probably be talked into removing some of these items from the list. There were some black lives matter protest marches in my city which I truly would describe as non-violent. I would unironically call them "mostly peaceful". There was a single case of looting at one store (there was more looting in nearby cities), but I would not let that tarnish the overall message of the movement. Probably most communities were like this. I would contrast this with a movement that openly endorses looting and property destruction as righteous forms of protest. I would also contrast this with a movement that denies any connection between the looting and the protest movement. Oddly I hear both kinds of excuse-making, often from the very same people. "It's not happening" or "It's a false-flag attack by right-wing activists" combined with "Looting and rioting are actually defensible modes of protest." This suggests a panicked effort to dismiss the problem rather than engage with it.
It's also useful to imagine a mirror image of this. Suppose there was a right-wing version of a street protest movement with a few violent actors. How does that narrative play out? How legitimate would the "mostly peaceful" label seem? Charlottesville 2017 comes to mind. Just think of how tone-deaf it sounds to refer to the white nationalists as "mostly peaceful," one of them having intentionally killed a counter-protester with his car. But imagine a less racially charged object of the protestors' outrage. Say it's a tax revolt of some kind, Tea Party 2.0. I seriously doubt the "mostly peaceful" modifier would survive a single act of violence, or the assault on or occupation of a single piece of government property. Or imagine a headline saying that protesters were beaten back by "mostly peaceful police officers." There is some kind of double standard at play here. In the case of phalanxes of police officers in riot gear confronting a crowd, in the case of well-armed right-wing militia men aggregating outside a government building, and in the case of left-anarchist mobs gathering at a building that's been the repeated target of arson, there is an implicit threat of violence. In all three cases, the "mostly peaceful" descriptor is a sick joke. It's conceivable that a righteous enough cause justifies the use of violence, threatened or realized. It's not conceivable that righteousness converts violence to non-violence. "Yep, we get to use violence because we're the good guys and they're the bad guys" would at least be the start of an honest discussion.