This is actually evidence against "violence is the only answer"

I don't think they argued at any point that violence is the only answer, so I don't think that that's relevant. Of course determining when violence is or is not the best answer is hard, and of course there are endless examples of people doing so wrong; I imagine that we can all agree on this. I also imagine that we can agree that there are scenarios where, in a utilitarian sense, violence is the best answer to stop other people from committing something that we see to be violence; whether we agree on the whole utilitarianism thing is a different issue, but there are other ethical philosophies that will countenance the same argument.