This was basically the question I asked on your other thread, Jock. How does a committed pacifist reconcile their belief with the reality of a justified war, such as that against Hitler, or, in this case, violence towards a loved one? There's a real moral and ethical dilemma here. Do personal principles and convictions take precedence at the expense of the innocent, or do we have a moral duty to protect those who are unable to protect themselves?
In the circumstances you describe, I'd hate to do it, but I know I would pull the trigger.
(Sorry I can't answer your question, but this is such an interesting subject, I found it difficult not to add my thoughts).