(A?)morality of nonviolence

This weekend we went to a conference at Hampshire College on animal rights, and before the conference there was a lot of sound and fury over Jerry Vlasak being invited to speak, because of the highly controversial statements he's made.

But this isn't a post about whether or not he should have been allowed to speak. This is, though, a post thinking about the things he said that got everyone all in a tizzy; namely, statements affirming the validity of employing violence as a tactic. If you google Jerry Vlasak you will find a lot of "Jerry Vlasak says to go kill scientists!" stuff, where what he actually said was that for people who can be convinced no other way to stop murdering animals, employing violence against them is morally justifiable. I know, I know, semantics: but it is an important distinction. Also important is to keep in mind context (which is inevitably left out), which is that the remarks were, just as this post, an academic discussion about tactics/ethics. Philosophizing about the morality of violence isn't at all the same as urging people to commit it.

But thinking about it brought to mind some questions that I've been discussing with people:

1) Is violence ever a morally justifiable tool?

2) If violence is ever a morally justifiable tool (e.g., self-defence, defence of innocent life/prevention of more violence), are there ever times it is amoral not to exert force?

3) (Which is really just reiterating the firsts in slightly more concrete terms): If there are things occurring such as murder, torture, etc., and you are in a position where you can stop it (by any means necessary; such as force if no other solutions are viable), and you do not do so, is that wrong? (If someone is standing in front of you with a gun to a child's head, and diplomacy has failed, is it wrong to take a sledgehammer to their head? Is it wrong not to, if you have the opportunity and you know that not killing the abuser will mean the death of the innocent party?)

Which all leads to question 4) If animal liberationists truly believe that animals have just as much right to live as the rest of us, that taking their lives is murder, then are they in violation of their own moral code if they fail to take necessary steps to prevent such killings?

I also would like to add that these questions have nothing to do with what anyone's views on animal lib are, and that clearly if animal liberationists use force to prevent the killing of animals, they are in violation of the law and probably in violation of a whole lot of other people's moral codes. But I'm not trying to answer the question of the sanctity of animal life here -- that's a whole other question. I am questioning whether or not, if someone truly believes in the sanctity of a life, does that equal ethical justification or even obligation to protect it, when the opportunity arises?

I'd also like to point out that I am not arguing one way or another about any of this. Construing questions as arguments assumes you know what my answers would be, and I assure you, you don't. I'm just thinking.

N.B., These questions apply equally to anyone who espouses belief in the sanctity of any life; the same question could be applied to pro-lifers.