Most movements develop an extremist faction, and this is how. Someone feels they are so morally superior that visiting violent 'justice' upon the 'evil' is the only to control people: never mind that historically violent assault results in violent retribution, it will only give them further vindication for their crusade.
You (typically) lose complete moral credibility when you advocate murder to save the lives of animals, but the truth is they're probably right: you can't reason your way out of it, but it's because they're completely loony, not because the 'vivisection complex' is some dark, forboding Illuminati-esque agency that must be decapitated like a serpent.
Edit: I ... should note that I'm not advocating violence against them when I say they probably can't be reasoned with. They're loony, but also tiny. Most people, even animal-rights activists are not terrorists. So there ought to be a healthy level of awareness, but truthfully it is likely best just not to engage them in either discourse or, this really goes without saying, with violence.