Political tradition has long called for the death of those who have committed heinous crimes; murder, forgery, and social disruption—all have had the penalties of death at some point in time (and in some cases, still do). But does such a penalty have any grounds to stand on? That is to say, if we look deeply into the matter, does the penalty of death for heinous crimes actually make sense?
Consider: for what purpose does the law impose death? Clearly, it does this to maintain order (which Plato tells us is the same as justice ). But, again, why maintain order? Well, order brings about expediency, which ensures our survival. In other words, when a state legislates just laws, criminals are sentenced to die in order that life may be preserved.
This sounds strange, but is a reasonable enough answer. Those who are killed are killed because doing so prevents the deaths of others—here the preservation of more life is the final end and true purpose of execution. Life here maintains value, is protected, and justice is maintained.
But what if the guilty party can reasonably be contained and prevented from killing again? Is there justice in their execution?
This brings us to the question of whether their life still has value—and so indeed whether life itself has value. For if the value of a life can be lost, it is not then an intrinsic property of the life itself, but an extraneous or accidental property of it, being then an intrinsic feature of that which must be attributed to the life in order for it to have worth. To illustrate and better explain my meaning, let us suppose that virtue is what gives life its value. If this is the case, when a life is without virtue—for example, because one feels no remorse for a mass murder they have committed—then it has no value, and consequently it was not the life itself which ever had a value, but merely the virtue which was ascribed to it. Most plainly put, if we suppose that any life has even the potential of having no worth, life itself necessarily has no worth.
But if life itself has any value, and the guilty is being executed on account of their destroying of that which has value (that is, not in order to protect that which has value, which has been recognized as just in the second paragraph), we unequivocally repeat their crime and increase its effect—we too destroy that which has value, and moreover have increased the net loss of life and its intrinsic value, all the while being entirely capable of preserving that object of value which we have just destroyed. Therefore—if we suppose life to have any worth—the death penalty as anything but a necessary means of preservation is an injustice of the kind for which the guilty is being killed.
And again we fall into a similar trap if we suppose that life is truly worthless and only accidentally with value—or at least this is so when considering reasonable suggestions as to what the provider of that external value of life is. Let us again return to the notion that virtue provides this value. Virtue, as defined by the General Doctor, is “a certain perfection of a power .” Now perfection lies properly speaking in the degree to which a thing exists , and so insofar as some power within a thing exists, it is virtuous. Therefore, under this notion of virtue granting worth, insofar as the guilty life still exists (and therefore too has its natural powers), it has worth, and so the execution of them falls into the trap of the above paragraph (i.e. we destroy that which has value, duplicating their crime and increasing its effect). Again, this will be true if we take any number of reasonable accidental attributes as the source of worth, because something has worth insofar as it is desirable, desirable insofar as it is perfect (or brings about perfection in some thing), and perfect insofar as it exists .
This means then, in summary of the above conclusions, that the execution of the guilty is just insofar as it is necessary to preserve life—in all other ways and situations it is absurd and unjust.
 Plato. Rep IV, 433b, trans. Jowett
 Aquinas. ST I-II. Q55. A1. trans. Fr’s OP
 Aquinas. ST I. Q5. A1. trans. Fr’s OP