Let’s assume that lethal self-defense is permissible. Such self-defense requires an aggressor. There is a variety of concepts of an aggressor for purposes of self-defense, depending on what constitutes aggression. Here are a few accounts:
voluntarily, culpably and wrongfully threatening one’s life
voluntarily and wrongfully threatening one’s life
voluntarily threatening one’s life
threatening, voluntarily or involuntarily, one’s life.
(I am bracketing the question of less serious threats, where health but not life is threatened.)
I want to focus on accounts of self-defense on which aggression is defined by (4), namely where there is no mens rea requirement at all on the threat. This leads to a very broad doctrine of lethal self-defense. I want to argue that it is too broad.
First note that it is obvious that a criminal is not permitted to use lethal force against a police officer who is legitimately using lethal force against them. This implies that even (3) is too lax an account of aggression for purposes of self-defense, and a fortiori (4) is too lax.
Second, I will argue against (4) more directly. Imagine that Alice and Bob are locked in a room together for a week. Alice has just been infected with a disease which would do her no harm but would kill Bob. If Alice dies in the next day, the disease will not yet have become contagious, and Bob’s life will be saved. Otherwise, Bob will die. By (4), Bob can deem Alice an aggressor simply by her being alive—she threatens his life. So on an account of self-defense where (4) defines aggression, Bob is permitted to engage in lethal self-defense against Alice.
My intuitions say that this is clearly wrong. But not everyone will see it this way, so let me push on. If Bob is permitted to kill Alice because aggression doesn’t have a mens rea requirement, Alice is also permitted to lethally fight back against Bob, despite the fact that Bob is acting permissibly in trying to kill her. (After all, Alice was also acting permissibly in breathing, and thereby staying alive and threatening Bob.) So the result of a broad view of self-defense against any kind of threat, voluntary or not, is situations where two people will permissibly engage in a fight to the death.
Now, it is counterintuitive to suppose that there could be a case where two people are both acting justly in a fight to the death, apart from cases of non-moral error (say, each thinks the other is an attacking bear).
Furthermore, the result of such a situation is that basically the stronger of the two gets to kill the weaker and survive. The effect is not literally might makes right, but is practically the same. This is an implausibly discriminatory setup.
Third, consider a more symmetric variant. Two people are trapped in a spaceship that has only air enough for one to survive until rescue. If (4) is the right account of aggression, then simply by breathing each is an aggressor against the other. This is already a little implausible. Two people in a room breathing is not what one normally thinks of as aggression. Let me back this intuition up a little more. Suppose that there is only one person trapped in a spaceship, and there is not enough air to survive until rescue. If in the case of two people each was engaging in aggression against the other simply by virtue of removing oxygen from air to the point where the other would die, in the case of one person in the spaceship, that person is engaging in aggression against themselves by removing oxygen from air to the point where they themselves will die. But that’s clearly false.
I don’t know exactly how to define aggression for purposes of self-defense, but I am confident that (4) is much too broad. I think the police officer and criminal case shows that (3) is too broad as well. I feel pulled towards both (1) and (2), and I find it difficult to resolve the choice between them.
