When is it permissible to kill in self-defence?

With regard to the first post on this blog, I'd like to raise for broader discussion the question of what constitutes material (as opposed to formal) aggression. The question can be posed without the jargon: when is it morally permissible to intend to kill someone in self-defence? One answer would be:
(A) It is morally permissible to intend to kill someone in self-defence only if that person is maliciously intending to kill (or cause grievous bodily harm to) one.
There are, however, some problems with this:
(i) it wouldn't allow one to kill in self-defence in war time (for the enemy soldiers wouldn't have malicious intent);
(ii) it also wouldn't allow one to kill in self-defence if one were about to be wrongly judicially executed (e.g. by mistaken identity);
(iii) it wouldn't allow one to kill in self-defence if one were attacked by someone acting in (what he or she thought was) *his or her own* self-defence;
(iv) it wouldn't allow one to kill in self-defence if one were subjected to grievous bodily harm by a doctor under the mistaken impression that he or she was performing a surgically beneficial procedure;
(v) most importantly, it wouldn't allow one to kill in self-defence if one were attacked by a madman or a drunk or someone under the influence of drugs;
(vi) also, it might well be that someone endangers my life maliciously without intending to kill me, e.g. if I am a shipwreck-survivor on a small plank in the open sea, someone tries to throw me off not to kill or hurt me but to save him or herself--I think that here it would be permissible to kill my assailant even though he or she doesn't intend to kill or hurt me.
At the other extreme would be this principle:
(B) It is morally permissible to intend to kill someone in self-defence only if that person would kill (or cause grievous bodily harm to) one if one weren't to kill that person.
But (B) seems too permissive for the following reasons:
(i) it would allow a mother to kill her foetus if it were endangering her life (though intended abortion is always condemned by the Vatican);
(ii) it would allow one to kill clumsy bystanders that were about to knock one in front of a bus;
(iii) it would allow one to kill a child that was playing with a handgrenade;
(iv) it would allow one to kill the person falling off a building on top of one.
Some of these cases are a bit tricky, and there is certainly controversy surrounding them.
I propose a midway principle:
(C) It is morally permissible to intend to kill someone in self-defence only if that person is about intentionally to kill (or cause grievous bodily harm to) one.
The point about (C) is that it allows one to kill people that are about intentionally to perform an action that amounts to killing or the causation of grievous bodily harm even if they do not intend to kill or cause grievous bodily harm. Thus, one may kill the person trying to tip one off the plank into the sea since he or she intends to perform the action of tipping one off, which will amount to killing or causing grievous bodily harm, even though the assailant doesn't intend to kill or cause grievous bodily harm. It will not, however, allow one to kill a child, to abort a foetus, or kill a clumsy bystander or a falling person, since they aren't intentionally about to perform an action that would amount to a killing or infliction of grievous bodily harm. It would allow one to kill a mad person or a drunk or someone under the influence of drugs provided that he or she was about intentionally to perform the action in question: there is a difference between a drunk's drunkenly pushing one over a cliff and a drunk's drunkenly falling into one knocking one over a cliff.

What do others think?