If a protest turns violent (people being attacked, property destroyed etc) that doesn't "spoil" the protest. The protesters can still be in the right for protesting something even if people on their side are being hostile. If people on your side are being violent that doesn't make you wrong all of a sudden. If that were the case almost every political affiliation would be in the wrong.
I'm not arguing against peaceful protest, nor am I advocating for riots or violence. I'm just saying that the protesters message matters more than the point their means of getting that message across. For example if an anti-fascist protest happens and a protester punches someone. The protest isn't instantly evil, to claim that it is would be ignoring the whole point of the anti-fascist protest in the first place.
All I'm saying is if someone does something bad in a protest, that doesn't make the protest bad.