This is the last post in the Thomas Says series on killing. I want to summarize some points Thomas makes in this question of the Summa.
First, let’s note the obvious, which hasn’t been noted yet in this series: The question of killing falls under the heading of “Justice.” In particular, it is in the section dealing with vices causing “injury to a neighbor against his will . . . by deed.” There are other vices of injustice that cause injury through words, both in legal and nonlegal contexts. Murder is an act that violates justice, which means that the murderer has failed to act in a way that promotes in himself (and others) the virtue of dealing correctly with his fellow human beings. Thomas also says that an unjust act (or a character disposed toward injustice) is opposed to the right.
A few of the conversations in the comments raised important issues. I want to focus on a very important one that was left open. The question is whether there are ever good grounds for killing another human being. In article 6 Thomas says, “If we consider a man in himself, it is unlawful to kill any man, since in every man though he be sinful, we ought to love the nature which God has made, and which is destroyed by slaying him.” This sentence of this quotation seems to mean that if we consider a human being only as an individual, there are no grounds to ever kill anyone. This is because “we ought to love the nature which God has made.” I think “the nature which God has made” is an allusion to the nature of a human being as an image of God. Similarly, in article 3 Thomas says that “a man who has sinned is not by nature distinct from good men; hence a public authority is requisite in order to condemn him to death for the common good.”
But if a person still retains their nature even after committing murder, in what way can anyone be justified in killing that person? Thomas’s view is that a proper authority can put to death a murderer because the murderer does not retain the dignity of his nature. This seems clearest in article 2: “By sinning man departs from the order of reason, and consequently falls away from the dignity of his manhood” and “although it be evil in itself to kill a man so long as he preserve his dignity, yet it may be good to kill a man who has sinned.” The implication in the latter quotation is that one who has sinned has failed to preserve his dignity.
This loss of dignity must be connected with the fact that the murderer harms the common good. For it is only in connection with considerations about the common good that Thomas thinks anyone is ever justified in putting another human being to death. He says, “the slaying of a sinner becomes lawful in relation to the common good, which is corrupted by sin.” In Thomas’s thought the maintenance of the dignity of one’s nature is connected with the goodness of commonwealth. The connection depends on his conception of what makes the common good good. The common good is good in part because the individuals composing it are good. In article 6 Thomas says that “the life of righteous men preserves and forwards the common good, since they are the chief part of the community.” I cannot maintain my dignity and go against the common good. Once I go against the common good, I lose my dignity. (As one commenter pointed out, we should be careful to understand what Thomas means by “common good.” His understanding of the common good is too large to be explained here, but suffice it to say that he is not a communist or crypto-communist.)