Violence is a bad thing. This basic truth—intuitive enough to most people that I don’t propose to attempt to prove it—is one of the essential elements of Just War Theory. Many people, including a lot of JWT advocates, think of it as a way of seeing how violent we’re allowed to be; but from a properly Catholic perspective, its function is to throw as many obstacles as possible in the way of the human impulse to resort to violence. The reason it is nonetheless called Just War Theory is that it admits there can be things even worse than violence, while still classifying violence as a very serious evil.
Violence is bad because life and health are good. We should want them for not only ourselves (a desire which our animal instincts normally ensure) but for our neighbor, i.e., human beings in general. Man was made as a being that has not only a spirit, like the angels, but a body, and the mysterious union of the two is the context and foreshadowing of the Incarnation, the union of God with matter. (1) Hence, any act of violence—any deliberate injury to the material body of another—is as terrible a thing as deliberately defacing an ikon; and more, for in this case the ikonographer is God. If your reason for resorting to violence wouldn’t be a good enough reason to swing an axe into the face of a painting of the Virgin Mary, it’s not a good enough reason to resort to violence. That is the mindset we need to approach any Catholic understanding of self-defense, including JWT.
Now, the Church does teach that violence in self-defense is acceptable, under strictly defined perameters, because everyone including oneself is an ikon of God. Justice to oneself is permitted, so to speak, as much as justice to others is commanded; the difference is that we are allowed to lay down our rights on our own behalf, but that isn’t a choice we’re entitled to make for other people. (2)
Those strictly defined perameters are ultimately related to JWT. The point of self-defense is to prevent as much harm as possible done by an aggressor. That means that you can’t hit first, because that makes you the aggressor, not them. (3) It means you cannot exact revenge through violence (though the impulse may be somewhat excusable). And it means you can only use as much force as you need to stop the aggressor, and no more.
Depending on circumstances, there can be cases in which only killing them can keep an aggressor from doing harm; this is why the Church has never absolutely forbidden either the death penalty or warfare. (4) No, not even under Pope Francis (forgive a mild digression), whose notorious “inadmissible” addition to the Catechism has always seemed to me like a judgment—quite a reasonable judgment—that modern facilities and technologies mean that the conditions which made the death penalty tolerable simply don’t exist any more. The use of the word “inadmissible,” as distinct from “wrong” or “intrinsically evil,” would seem to suggest that interpretation. It may not have been practicable in the past to permanently and securely house murderers and terrorists, and in those circumstances, the only way to keep the rest of the community safe may well have been to resort to the death penalty. It’s not like that now. Circling back to that “depending on the circumstances” above, it does precisely depend on the circumstances: when they’re such that something other than killing or harming an aggressor can keep them from doing harm, killing or harming them is going too far, it’s using excessive rather than proportional force, and the person who does that bears moral guilt for doing so.
This is not appealing to all palates. Most of us want to hit back when we’ve been hurt, physically or otherwise. And many of us feel that fighting for Our Side, whether defined in terms of country or ethnicity or party or, yes, religion, ought to justify using any means necessary for Our Side to win. It doesn’t. Not on a Catholic view, anyway. No side is God, and, accordingly, no side is guaranteed to be unconditionally in the right without qualification in every dispute; even the Church isn’t, since it is her teaching office alone and not the total conduct of her ministers which is protected from error.
And when you consider that part of Catholicism is the belief in the Last Judgment, where literally everything will be set to rights, that makes a lot of sense. We aren’t able to correct everything, we aren’t allowed to correct everything, and we don’t need to correct everything. “All we have to do is decide what to do with the time that is given to us.”
(1) There is an unusual but orthodox theory among Catholics that the Incarnation was the original purpose of creation, rather than being decided on by God as a sort of response to human sin.
(2) There is an exception here if you’re responsible for the life and well-being of other people. In that case, defending your own life can actually rise to the level of a duty, because in that case surrendering your rights would precisely be surrendering other people’s rights too.
(3) You could, say, catch a guy’s fist as it swung toward you; but hitting first because someone looked like they might be going to resort to violence is, at best, a self-fulfilling prophecy prompted by panic, not an instance of wisely and correctly applied self-defense. This is one of the many reasons that eleven out of ten appeals to “preëmptive strikes” as a justification for military action are nonsense and/or lies.
(4) That said, the Church before Constantine did forbid Christians to join the military, and soldiers who converted were ordered to seek non-violent roles like cook or medic until they could be discharged. Moreover, from the Christianization of the Roman Empire until the Crusades, it was standard practice to assign penances to soldiers, even (apparently) if they had been fighting in more-or-less just and defensive wars (as in the Spanish and Frankish defenses against the Umayyads in the eighth century). This may have served the function of trying to ameliorate the trauma of participating in violence—something that affects people no matter how justifiable their actions were.