I’ve always liked Hans-Hermann Hoppe’s observations regarding how we have to treat aggressors as technical, not ethical, problems. From The Economics and Ethics of Private Property (relevant parts bolded):
while scarcity is a necessary condition for the emergence of the problem of political philosophy, it is not sufficient. For obviously, we could have conflicts regarding the use of scarce resources with, let us say, an elephant or a mosquito, yet we would not consider it possible to resolve these conflicts by means of proposing property norms. In such cases, the avoidance of possible conflicts is merely a technological, not an ethical, problem. For it to become an ethical problem, it is also necessary that the conflicting actors be capable, in principle, of argumentation.
Whether or not persons have any rights and, if so, which ones, can only be decided in the course of argumentation (propositional exchange). Justification—proof, conjecture, refutation—is argumentative justification. Anyone who denied this proposition would become involved in a performative contradiction because his denial would itself constitute an argument. Even an ethical relativist must accept this first proposition, which has been referred to as the a priori of argumentation.
From the undeniable acceptance—the axiomatic status—of this a priori of argumentation, two equally necessary conclusions follow. First, it follows under what circumstances no rational solution to the problem of conflict arising from scarcity exists. Suppose in my earlier scenario of Crusoe and Friday that Friday was not the name of a man but of a gorilla. Obviously, just as Crusoe can run into conflict regarding his body and its standing room with Friday the man, so he might do so with Friday the gorilla. The gorilla might want to occupy the same space that Crusoe occupies. In this case, at least if the gorilla is the sort of entity that we know gorillas to be, there is in fact no rational solution to their conflict. Either the gorilla wins, and devours, crushes, or pushes Crusoe aside (that is the gorilla’s solution to the problem) or Crusoe wins, and kills, beats, chases away, or tames the gorilla (that is Crusoe’s solution). In this situation, one may indeed speak of moral relativism. One may concur with Alasdair MacIntyre, a prominent philosopher of the relativist persuasion, who asks as the title of one of his books, Whose Justice? Which Rationality?—Crusoe’s or the gorilla’s? Depending on whose side one chooses, the answer will be different. However, it is more appropriate to refer to this situation as one in which the question of justice and rationality simply does not arise: as an extra-moral situation. The existence of Friday the gorilla poses for Crusoe merely a technical problem, not a moral one. Crusoe has no other choice but to learn how to manage and control the movements of the gorilla successfully just as he must learn to manage and control the inanimate objects of his environment.
By implication, only if both parties to a conflict are capable of engaging in argumentation with one another can one speak of a moral problem and is the question of whether or not there exists a solution meaningful. Only if Friday, regardless of his physical appearance (i.e., whether he looks like a man or like a gorilla), is capable of argumentation (even if he has shown himself to be so capable only once), can he be deemed rational and does the question whether or not a correct solution to the problem of social order exists make sense. No one can be expected to give an answer to someone who has never raised a question or, more to the point, to someone who has never stated his own relativistic viewpoint in the form of an argument. In that case, this “other” cannot but be regarded and treated like an animal or plant, i.e., as an extra-moral entity. Only if this other entity can in principle pause in his activity, whatever it might be, step back so to speak, and say “yes” or “no” to something one has said, do we owe this entity an answer and, accordingly, can we possibly claim that our answer is the correct one for both parties involved in a conflict.
Likewise, for a human who refuses to engage in rational discourse, who refuses to recognize and respect the basic rights of others, they must also be regarded as technical problems and dealt with as if they are wild beasts. As I noted in Quotes on the Logic of Liberty, this is a common sense ethical truth that many others have observed, such as John Locke and others, as indicated in the quotes below:
“In transgressing the law of Nature, the offender declares himself to live by another rule than that of reason and common equity . . . and so he becomes dangerous to mankind; . . . every man . . . by the right he hath to preserve mankind in general, may restrain, or where it is necessary, destroy things noxious to them, and so may bring such evil on any one who hath transgressed that law, as may make him repent of the doing of it . . . .” B6 11: A murderer “hath, by the unjust violence and slaughter he hath committed upon one, declared war against all mankind, and therefore may be destroyed as a lion or a tiger, one of those wild savage beasts with whom men can have no society nor security.” —John Locke, The Second Treatise on Civil Government B6 8
“With him [an aggressor] we are returned to the first-stage state of nature and may use force against him. In so doing we do not violate his rights or in any other way violate the principle of right, because he has broken the reciprocity required for us to view such a principle [of rights] as binding. In this we find the philosophic grounding for the moral legitimacy of the practice of punishment. Punishment is just that practice which raises the price of violation of the principle of right so as to give us all good reason to accept that principle.” —J. Charles King, A Rationale for Punishment, 4 J. Libertarian Stud. (1980): 154.
“[J]ust as one cannot win a game of chess against an opponent who will not make any moves–and just as one cannot argue mathematically with a person who will not commit himself to any mathematical statements–so moral argument is impossible with a man who will make no moral judgements at all . . . . Such a person is not entering the arena of moral dispute, and therefore it is impossible to contest with him. He is compelled also–and this is important–to abjure the protection of morality for his own interests.” —R.M. Hare, Freedom and Reason (1963): A7 6.6
“To argue with a man who has renounced the use and authority of reason, and whose philosophy consists in holding humanity in contempt, is like administering medicine to the dead, or endeavoring to convert an atheist by scripture.” —Thomas Paine, The Crisis No. V
“When men hire themselves out to shoot other men to order, asking nothing about the justice of their cause, I don’t care if they are shot themselves.” —Herbert Spencer, from Facts and Comments (1902)
“[T]he victim is entitled to respond according to the rule (‘The use of force is permissible’) that the aggressor himself has implicitly laid down.” —John Hospers, “Retribution: The Ethics of Punishment,” in Assessing the Criminal: Restitution, Retribution, and the Legal Process, Randy E. Barnett and John Hagel III, eds. (Cambridge: Ballinger, 1977): p. 191.
“The injury [the penalty] which falls on the criminal is not merely implicitly just–as just, it is eo ipso his implicit will, an embodiment of his freedom, his right; on the contrary, it is also a right established within the criminal himself, i.e., in his objectively embodied will, in his action. The reason for this is that his action is the action of a rational being and this implies that it is something universal and that by doing it the criminal has laid down a law which he has explicitly recognized in his action and under which in consequence he should be brought as under his right.” —G.W.F. Hegel, The Philosophy of Right A7 100 (T.M. Knox trans., 1969) (reprinted in Philosophical Perspectives on Punishment (Gertrude Ezorsky ed., 1972): 107 (Emphasis in last sentence added; brackets in Ezorsky)
“[W]hen someone is punished for having violated others’ rights, it is not the case that the criminal has alienated or otherwise lost his rights; rather, it is the case that the criminal’s choice to live in a rights-violating way is being respected.” —Douglas B. Rasmussen & Douglas J. Den Uyl, Liberty and Nature: An Aristotelian Defense of Liberal Order (1991): 85
“[I]f someone attacks another, that act carries with it, as a matter of the logic of aggression, the implication that from a rational moral standpoint the victim may, and often should retaliate.” —Tibor R. Machan, Individuals and Their Rights (1989): 176
Update:“… one might ask, how does one answer an opponent who says: ‘You’ve demonstrated that I must accept your axioms if I am to be consistent. But that demonstration rests on your axioms, which I don’t choose to accept. Tell me why I should. Why can’t I contradict myself?’
“There is only one answer to this: stop the discussion. Axioms *are* self-evident; no argument can coerce a person who chooses to evade them. You can show a man that identity is inescapable, but only by first accepting the fact that A is A. You can show that existence is inescapable, but only by accepting and referring to existence. You can show that consciousness is inescapable, but only by accepting and using your consciousness. Relying on these three axioms, you can establish their position as the foundation of all knowledge. But you cannot convince another person of this or anything until he accepts the axioms himself, on the basis of his own perception of reality. If he denies them, it is a mistake to argue abot or even discuss the issue with him.
“No one can think or perceive for another man. If reality, without your help, does not convince a person of the self-evident, he has abdicated reason and cannot be dealt with any further.”
—Leonard Peikoff,“Sir, I have found you an argument; but I am not obliged to find you an understanding.” —Samuel Johnson