An eye for an eye makes the whole world circumspect. But, also, an eye for an eye makes the whole world cooperative, as Robert Axelrod showed with his study of tit for tat and competing strategies using iterated rounds of contests among genetic algorithms (described in his book The Evolution of Cooperation). Tit for tat beat all the alternative strategies, again and again; and as rounds of the contest were iterated, with winning strategies favored by the reproductive mechanism of the iteration, it more and more perfused the population of competing algorithms. As tit for tat increased in frequency, so did the total value generated by all competitors in each round: fewer and fewer defections occurred, and responses to defections were more and more often optimal.
That tit for tat wins the evolutionary game does not mean that its superiority is merely adventitious, an artifact of this or that sequence of random events that might have been quite different, and so generated quite a different sort of winner. On the contrary: provided the game goes on long enough, tit for tat wins every time, sooner or later, and no matter how the sequence of outcomes varies. The utile superiority of tit for tat is a truth of game theory, so that like any other mathematical truth it is from before any and all worlds, and holds true in every world. The metaphysical superiority of tit for tat, then, is the source and reason of its practical evolutionary success, and not vice versa (this is true of all perdurant evolutionary success). Tit for tat is the optimal strategy in evolutionary practice because it is the best in metaphysical fact. As metaphysically best it is the most moral policy of all (these are two ways to say the same thing).
Tit for tat is just the Law of Compensation – Justice, Karma, Rta, the Tao – carried into practice in the operations of creatures. As mathematical, its morality is built into the universe from before the get-go. This is to say no more than that the Law of Compensation is a subsidiary of the Logos; so that morality is ever occasional or, as reiterated, conventional, only because it is first and originally eternal, and absolute. Particularities of the moral law, that pertain to this or that cultural or social predicament, are never therefore “merely” conventional, as only adventitious, and so specious, thus disposable. Adventition is not eo ipso irreal or arbitrary, or irrational. Adventitious particularities can after all attain concrete actuality only as rational instantiations of variations upon absolute eternalities.
As the original and basic Fact, God is in the final analysis then himself the compensation for all creaturely acts, and the answer to all their proposals; for, he judges and repays them in virtue of his mere existence (their existences being all responses to his); and tit for tat is part of who he is, what he thinks, and how he works. It is a member of the infrastructure of all things. So it is one of the ways God works on, in and for the world; or as we might more accurately put it, tit for tat is an aspect of how the world runs on God, and under him.
There is no other way things might be run. Reality himself cannot then possibly be mocked; so all mockery redounds to the mocker. Vengeance is mine, saith the Lord; I will repay. Thus is tit for tat everywhere efficacious, and all the proper balances and harmonies among creatures ultimately maintained, howsoever mediated through manifold causal chains among creaturely agents, such as judges, princes, kings, or other accidents of history.
Tit for tat, then – an eye for an eye – is the basic form of all cooperation among agents, and is therefore the very foundation of social order. Not just of human social order, either, but of social order per se. Even host/parasite relations tend toward symbiosis: mutual back scratching, exchange, coinherence. Viz., the symbiosis of the kshatriya, brahmana, vaisya, and sudra. Indeed, even the social order of subatomic particles may be treated as a variety of tit for tat. Every coordination is a type of charity, and every transaction wherein values of any sort are conserved is a mutual donation.
Notice then that worlds as such are coordinations of actualities. As the basic form of social coordination among entities, tit for tat makes worlds.
Thus any attempt to weaken the law is effectually an assault, not just on the social order of a given time or place, but on society as such; and on reality; and on Reality. Such assaults cannot but redound, somehow. They are logically autophagous. Vitiate your laws, and you vitiate your society, reducing its capacity to continue with its vices in the face of the corrections that always arrive without remit from elsewhere.
None of this is to say that there can be no such thing as mercy in sentencing (or as love of any sort, in any relations among creatures). Mercy is not the opposite or deformation of Justice, but rather its proper adjustment to particular situations, so as to be just in respect thereto, mutatis mutandis; for, no finite policy can comprehend all circumstances in all their relevant detail. This is why judges are furnished with sentencing guidelines, but enjoy wide latitude in their application.
Judges can be merciful; Justice herself cannot. You can’t make an exception to a rule that isn’t operant in the first place. So Justice herself must be implacable, in order for there to be any ontological capacity for mercy on the part of the judge. The system of justice, then – the jurisprudential rules, protocols, and guidelines – cannot itself be made a means of the delivery of mercy, without wrecking the whole shooting match.
 “Mercy” is from the Latin mercedem, “reward, wages, pay hire,” from merx, “wares, merchandise.” It is in the same family of terms as merchant, market, Mercury, and mercenary. Thus mercy is what is justly due to a man from the court, given a full comprehension of the circumstances. It is, i.e., not different from justice. And, justice applied to particular circumstances takes always the form of love, howsoever obscured by severity, or grief, or wrath.