It’s immoral to kill an innocent human being. That’s because we all have a “right to life”—a moral claim on one another not to be killed.
But some might say that this approach creates a conflict with our general intuitions about justified lethal self-defense. Does the right to life extend even to an aggressor whose behavior will kill me (and I have no other means to effectively preserve my life)? It would seem so.
Think about it: if every human being as a right to life, and the aggressor is a human being, then the aggressor has a right to life. To deliberately kill him in self-defense, therefore, even if there is no other means of saving my life, would seem to be just as much a violation of justice as would be the deliberate killing of an innocent human person. And if that’s true, then it would be immoral to deliberately kill the aggressor.
For most of us, that doesn’t seem right. It runs contrary to our common intuitions. But long held intuitions are being washed away with the tides of modern thought—so we need to ground our intuition in something more stable. So why is it morally permissible to kill in self-defense?
We can start with an idea that we’ve looked at elsewhere: equality with other human beings in behavior that’s naturally consistent with the exercise of life, called the “equality of relations” (Summa Theologiae II-II:79:1), is naturally due to human beings. In other words, I owe it to you not to kill you—to be innocent in my behavior toward you. The same applies the other way around. St. Thomas Aquinas calls this the “equality of justice” (ibid).
Here’s where the rubber hits the road when it comes to self-defense. The obligation not to kill arises from an order of relationship that requires not only that we be innocent in will (what philosophers call “formal innocence”), but also that we be innocent in behavior (“material innocence”). When an aggressor attacks me with a behavior that by nature is going to kill me (even if the behavior is not voluntary, like in the case of a mentally crazed person), assuming that I didn’t attack him first, his behavior is no longer innocent. It has disrupted the equality in behavior that nature demands—in particular, the behavior that’s naturally consistent with the exercise of life. This being the case, the behavior is outside the order of the “equality of relations” that nature requires for the “equality of justice” and therefore is defective or disordered. How can I owe him anything then? The “equality of justice” rises or falls with the “equality of relations.”
Consider, for example, a father who tells his son, “Go into the store and steal me a beer!” Must the son obey? Absolutely not! Why? Because the father’s command is outside the proper order that nature requires for a father’s command—an order where the command directs his son to do good and avoid evil for his perfection as a human being.
And so, just as the son doesn’t owe obedience to the father’s disordered command, I don’t owe behavior that’s naturally consistent with the exercise of life as a response to the aggressor’s disordered act of aggression (an act of the kind that kills). In other words, it seems that I can defend myself by deliberately killing him without violating justice.
Not only does this seem so. It must be so. Why? To say otherwise would entail nature being defective with regard to necessary things. It would be self-defeating (Summa Contra Gentiles, 3.129).
Consider that if the aggressor’s right to life were so strong that I couldn’t kill him in the above scenario, nature would be practically safeguarding the aggressor’s behavior that thwarts the natural order for his life as a social animal. On this supposition, nature says that I can’t stop him. Remember: in this scenario there’s no other means for me to stop the behavior other than a lethal blow, and we’re assuming here that there’s no proper authority to turn to in the moment. And so there would be no one to stop the aggressor. That’s a self-defeating move: directing a human being to pursue his perfection as a social rational animal but also safeguarding him thwarting that perfection.
Also, the whole purpose of nature’s demand for another human being to refrain from killing me is to protect my life. If nature forbade me to kill the aggressor in the above scenario, then nature’s design would involve a space where there is no possibility for the protection of my life. That’s also self-defeating: setting out to protect my life while at the same time demanding that my life not be protected.
Someone might counter, “Well, there’s the possibility of proper authorities protecting your life.” But what if it’s those in authority who are trying to unjustly kill me? In this scenario, there would be literally no possible way to protect my life. My right to life would become a “duty to die.” And this would be due to nature’s design, which would be absurd.
Bottom line: it’s self-defeating for nature to give us a natural right we can’t protect. Philosopher Timothy Hsiao sums it up nicely: “If I possess the right to life, then I must also possess the corresponding right to secure or protect my life.”
Now, this doesn’t mean that I can kill an aggressor in any circumstance where his behavior violates the “equality of relations.” What I owe him (or don’t owe him) will depend on the degree of the inequality he creates with his attack.
For example, if the aggressor’s attack is such that it only limits my use of some good—e.g., he tries to steal my iPhone—I’m not thereby justified to kill him. The relation is unequal only with regard to the free use of personal goods—something that’s pretty far removed from the good of life. (Although it wouldn’t be just to kill him in order to get my iPhone back, it would be just to wrestle him to the ground [ST II-II:41:1].)
In other words, my defense must be proportionate to the inequality caused by the attack. As Aquinas puts it, “an act may be rendered unlawful, if it be out of proportion to the end” (ST II-II:64:7).
We started with the question, “Does the right to life extend even to an aggressor whose behavior lacks innocence to the degree that it’s not naturally consistent with the exercise of life?” As we’ve seen, human nature says no! The right to life extends only as far as nature allows it.
Nature sets boundaries that circumscribe a moral space in which another human being can rightly demand, in justice, that I not kill him—it’s a space of innocence, a space where there exists an equality of behavior that’s naturally consistent with the exercise of life. But those same boundaries reveal nature’s design for what I don’t owe the other person—namely, a duty to die.
So nature has given us a moral recipe for killing. Deliberately killing an innocent human being is an injustice, and therefore immoral. Deliberately killing an aggressor whose behavior will kill me, when there are no other means to preserve my life, is not an injustice—and, therefore, it’s morally permissible, and in some cases obligatory. Self-defense, therefore—even lethal self-defense—certainly can be compatible with the right to life.