In late 1991 I was part of the evening security team at Golden Gate University in downtown San Francisco. One night shortly after I left (around 9 in the evening), people protesting the Gulf War tried, unsuccessfully, to break into the lobby. The next night I stayed late, and my soon-to-be-wife did, too.
After it was clear that the protestors were not coming our way, but going up Market Street, we left and joined the march.
At one point, a guy ran toward the plate glass front door of an office building with a sizable rock or chunk of pavement and threw it. The security guard inside looked terrified, but fortunately the rock did not break the glass, and even more fortunately, other protestors quickly grabbed the guy and said "we're here to protest violence, not engage in it."
Protests will bring out a variety of people, and because of that variety, in an important way there is no such thing as "the" protestors. Some are intent on peacefulness. Some believe violence is a solution. Some are just bad actors who are opportunistic and see an opportunity to get away with violence and theft that they'd like to anyway.
The problem is not "the" protests: protesting is justified. The problem is not "the" protestors: most are non-violent. The problem is it's hard to protest without drawing in some assholes.
But if you pay attention, you'll see numerous cases of peaceful protestors trying to police the protests, to constrain the bad actors, as I saw in the anti-Gulf War protest. Those people are heroes, in their own small way, and we should notice and honor them.