The question of whether a cleric can kill a sinner (criminal) is not one that occupies the thoughts of Christians today. But whenever we find such a question that occupies a past thinker it’s important to reflect on why we don’t find such questions pressing. Sometimes we don’t find the question pressing anymore because the past thinker has settled the question decisively. Take, for example, the question of whether Christians should read and theologize about the Hebrew Bible (the Old Testament). This isn’t a pressing question today among Christians partly (perhaps primarily) because Augustine settled the issue in The City of God. Similarly (perhaps), we don’t think the question of whether it’s lawful for a cleric to kill a sinner as pressing because Aquinas (at least partly) settled the issue.
So, what are the reasons to think that it might be lawful for a cleric to kill a sinner? First, you might think that since Christians are supposed to imitate God, and since God killed sinners, clerics who want to imitate God should kill sinners. You might also think that since the secular authorities are inferior to the clerical authorities, and since the secular authorities put sinners to death, the clerical authorities should put even more people to death than the secular authorities. Or you might think that in cases where clerical authority and secular authority coincide in one person, that person could kill sinners.
Truthfully, you might not think any of these things. But these are the objections Thomas addresses.
Here’s how Thomas responds. He quotes from the apostle Paul. He’s working with the Vulgate, so he states that a “bishop should be blameless . . . not a striker” (1 Tim. 3:2–3). Then he gives two reasons to think that this criterion for clerics makes good sense. First, that clerics in the “ministry of the altar” should be imitators of Christ, who did not return blows when he was struck. Second, that clerics are ministers of the “New Law” according to which “no punishment of death or of bodily maiming is appointed.” Thomas points out that the priests and Levites of the Old Testament were ministers of the Old Law, so it was lawful for them to kill sinners with their own hands since the Old Law had provisions for corporal punishment.
With these reasons in hand, Thomas addresses the objections you might have. First, it’s true that Christians should be imitators of God, but each Christian should imitate God “in that which is specially becoming to him [the Christian].” I don’t know what is specially becoming to you, but (even as a noncleric) I find this argument convincing. Second, it’s true (again) that clerical matters are higher than secular matters, but this doesn’t mean that clerics should express their superiority by doing a super duper job in secular matters. It means instead that clerics should leave the minor matters to secular authorities and not concern themselves with what’s beneath them. Last, in cases where spiritual and secular authority coincide in one person, that person should not punish sinners with his own hands. Others should punish sinners in virtue of the cleric’s authority.
I find the most interesting part of this discussion to be Thomas’s understanding of the Old and New Laws. Specifically, he thinks that the New Law does not contain any provision for human, spiritual authorities to carry out corporal punishment. If a Christian layman is not being a good Christian (in some respect or other), his priest or pastor has no right by the New Law to corporally punish him. Contrast with the Old Law where, Thomas thinks, corporal punishment for spiritual matters seems a live option. (Question for Old Testament scholars: Is Thomas right about this?)
How well does Thomas’s reasoning transfer to those ministers outside the Roman Catholic tradition? If Thomas is right about what the New Law enjoins and forbids, then I think his reasoning transfers quite well. Protestant ministers, for example, see themselves as ministers of the New Law, so they should also consider that as ministers of the New Law they should not corporally punish sinners. I suppose a question could arise about the phrase “as ministers.” Suppose a Protestant pastor is both a pastor and, say, a police officer. (Anyone know of such an instance?) Why not say that the person punishes as a police officer? I think Thomas would say that such a person ought not be both a pastor and a police officer because the office of a pastor should not be confused with (that is, mixed together with) other offices. Why not? Probably because of the sanctity of the pastor’s office.