I don't think anyone really has a moral responsibility other than to themselves, although they have a moral 'contract' with society. Saying this I'm not suggesting that we should all be selfish - it is my conviction that we are actually happier people when we are less egotistical and self-serving.
Thus to talk of defending or not defending as being right or wrong is missing the point - (in my opinion) we should act from the belief that the universe is compeletly interconnected and is actually one whole.
Personally, I would (and have) stopped someone attacking someone else, as well as attacking me. I would try to refrain from injuring an attacker, since I don't believe this would encourage them to desist in their behaviour (similarly, in christianity, it is better to change people by example rather than through violence - people usually just resent violence).