I think it's very unfortunate that it started out as a peaceful protest and got violent. Seems like the peaceful protestors would try to put pressure on those of them who take advantage of the moment to plunder and destroy.
It just takes credibility away from their protest.
It's very hard to tell, often violence is reported when little actually exists (yea it's where filming comes in to collaborate reports, and even then you need the full footage to see if anything incited it or not). Since protests are often infiltrated, it could just be provacators inciting violence (most protestors expect the possibility of infiltration). And it also could be da da dum: some random no-goodnics who have decided to come to the protest (no I'm not going to call them "the protestors") getting violent. But it would seem to me if one wanted to confront violent people (which is what your arguing protestors somehow have a responsibility to do, even though most probably have no training in doing so) one would have to know what they were doing - are they likely to listen? Are they even there for "the cause" in some sense or for their own purposes? So I'm saying even if it's nogoodnicks, it actually in NO WAY takes credibility away from the protest and all the peaceful protestors that are there. It's irrelevant to that.