▻ THE DIVERSITY OF ETHICAL THEORIES
There are different ethical perspectives from which your question can be answered. This is clear from the first two replies. I suggest an approach from the side of rights. We are not all Kantians or utilitarians or signed up to virtue ethics but we (nearly) all use the concepts of rights and justice. It enters into virtually all our everyday moral arguments. I might add that it is a part of ordinary moral thinking that there are times when it is permissible intentionally to kill another person in self-defence. What principled moral defence from the viewpoint of rights can this axiom be given ?
We need an argument, not a statement of general positions, to see what the basis of the moral right of a potential victim to kill a morally innocent 'aggressor' - an innocent source of lethal danger - actually is or might be.
▻ ARGUMENT 1 - JUDITH THOMSON ON SELF-DEFENCE
Judith Thomson tries to define the basis as follows :
According to [Thomson], if the aggressor holds no right to kill you (i.e., his aggression is not a response to a lethal threat you pose to him), you may kill him just because otherwise he would kill you.Hence, killing an aggressor is morally justified, even if he is free of fault in committing the particular aggression that threatens your life. Culpability has nothing to do with the right to self-defense, since an agent who exercises this right is not an agent of justice. The major argument Thomson advances appeals to the presumption against exercising private violence (I call this presumption the Weberian Presumption*). She takes it as almost self-evident that the Presumption cannot be defeated either by the aggressor's being liable to harm, or by his deserving
it. ... She infers that it is defeated simply because the aggressor is about to kill someone and has no right to do so. ( Yitzhak Benbaji, 'Culpable Bystanders, Innocent Threats and the Ethics of Self-Defense', Canadian Journal of Philosophy, Vol. 35, No. 4 (Dec., 2005), p.586.)
[*'Weberian' refers to Max Weber's view of the state as having a monopoly of the exercise of legitimate violence. The 'presumption' is against the exercise of legitimate violence being 'privatised', transferred to an individual.]
What are we to make of Thomson's argument ? Let's read Benbaji a little more :
Thomson's basic claim is simple: you have the right to kill an aggressor
in self-defense if he would otherwise kill you (while he holds no right to
do so). Indeed, you have this right, because otherwise the aggressor
would kill you. But what if the person whose being killed is necessary
for preventing your death became an aggressor through no fault of his
own? For example, what if the aggressor hallucinates that you are about
to kill him? What if he is under the influence of drugs that cause him to
be extremely aggressive? Furthermore, imagine that the one who is
about to kill you is not an aggressor at all. You are sitting on a bench, and
due to some cause, a man is falling on you - someone pushed him, or
he lost his balance, or whatever. It turns out that, unfortunately for both
of you, unless you blow him to pieces with a gun you have handy, he
will crush you. Do you have the right to defend yourself against such an
innocent threat? Thomson answers affirmatively. You are allowed to kill
anyone who would otherwise kill you. True, the falling man won't kill
you by any action of his, but still, he would kill you the way a stone can
kill you - by falling on you. And he has no right to do so. (Benbaji, p.587.)
▻ PROBING ARGUMENT 1
The argument generalises. If I have the moral right to kill in the situation described (the gun case) then so has anybody else similarly situated. It is not a private right.
The argument does not cover all cases of killing an innocent person for the sake of self-preservation. If someone is spraying bullets at me, the argument does not justify my grabbing an innocent person and using them as a shield.
Yet a serious objection to the argument seems to be this. Suppose that in the gun case I could only kill the innocent aggressor by firing a gun that would detonate a bomb that would (a) kill a large number of other innocents and (b) also kill me. These are situational factors that Thomson's argument does not consider. It is hard to see how her self-defence argument would pass the test of these extra factors.
▻ ARGUMENT 2 - THE RIGHT TO PROTECT ONE'S INTERESTS
If I kill through murderous aggression, then I kill disrespectfully. I am willing to accept that Kantian considerations of respect for persons apply here. Equally if I protect myself from murderous aggression I am protecting myself from the intention of being disrespectfully killed. Here also I recognise the relevance of Kantian considerations. I see the right to kill an innocent person in self-defence in terms of the right to protect my most basic interest, that of staying alive. Specifically, the right to kill an innocent threat, X, derives from the unfortunate fact for X, that X has infringed my claim against X that X not threaten my most basic interest (staying alive). A Kantian might note that though this argument is not based on respect for persons, it is capable of universalisability.
J. Thomson, 'Self-Defense', Philosophy and Public Affairs 20 (1991), 283-310.
Y. Benbaji, 'Culpable Bystanders, Innocent Threats and the Ethics of Self-Defense', Canadian Journal of Philosophy, Vol. 35, No. 4 (Dec., 2005), 585-622.
J.H. Beale, 'Homicide in Self-Defence', Columbia Law Review, Vol. 3, No. 8 (Dec., 1903), 526-545.