Previously I covered what Thomas says about killing plants and animals and killing sinners. Regarding plants and animals, Thomas says it’s okay to kill them because they are for our use. Regarding sinners, Thomas says it’s okay to kill them because they have by their actions descended to the level of animals—sinners have lost their human dignity. He also says that it’s permissible to kill sinners because they threaten the health of the community, and any part that threatens the health of the whole may be excised in order to safeguard the whole.
We progress now to the next article: Whether it is lawful for a private individual to kill a man who has sinned. (Killing in self-defense is a question Thomas takes up later. So let’s put that aside for now.) Here’s one reason you might think that a private person (as opposed to a person invested with public authority) could lawfully kill a sinner: Sinners are equivalent to animals, and anyone can kill an animal, especially if it’s dangerous. Here’s another reason: Protecting the common good is praiseworthy; so if a private person kills a sinner, he deserves to be praised.
Thomas won’t accept either of these reasons. In fact, a private person who kills a sinner is a murderer and worse: he’s also a usurper of legitimate governmental power. Why? Only those who have been entrusted with the care of the community can legitimately take action against threats to the community. Thomas gives the example of a physician who amputates: Only the physician can legitimately amputate because only the physician has been entrusted with the health of the body. Similarly, only those “persons of rank having public authority” have been entrusted with the health of the public good, so only they can kill sinners.
Now, about that suggestion that anyone can kill a sinner because sinners are animals and anyone can kill an animal. I can’t tell what distinction Thomas is relying on to make his point. (Thomas almost always makes a distinction in settling a dispute.) First, he distinguishes between wild animals and domestic animals. No one needs permission to kill a wild animal, but if one kills a domestic animal without authority, then one has to compensate the owner for the loss. Second, he distinguishes between a sinner’s being distinct from other human beings “by nature” and being distinct from them in that the sinner no longer has his human dignity. A sinner is still a human being by nature even though he has taken action (freely) to destroy his human dignity. (The distinction between being a human being by nature and maintaining one’s human dignity was made in the reply to the third objection in the previous article on killing sinners.) From these distinctions Thomas concludes that “a public authority is requisite in order to condemn him [the sinner] to death for the common good.”
It seems to me that Thomas thinks of a sinner as somehow being on the level of a domestic animal, not a wild animal. Perhaps he thinks that in virtue of being a human being by nature, the sinner is still part of human society and is entrusted to the care of the civil leaders, much like a domestic animal is cared for by its owner. So if someone harms the sinner without the proper authority, then he has gone against the (rightly appointed) civil authorities.
Thomas’s response to the other objection is clearer: You can do anything to support the common good provided your action does not harm another. If your action in support of the common good will harm someone else, then it must be approved of by the one to whom the care of the health of the society is entrusted.
We should note that in this section Thomas has not stated the conditions for someone being a legitimate authority. He supposes that there are (or can be) legitimate authorities in a commonwealth. I don’t know enough about his political views to know what he would think about the authority of citizens in a democratic republic. However, what he does say seems to rule out the vigilantism of, say, Batman. (Though perhaps he might say that Batman (at least in most versions of the story) is not, strictly speaking, a vigilante because he has at least the tacit approval of Commissioner Gordon, who is a legitimate authority.)
