A thug comes into your store and pulls a gun on you. A customer in the store is packing heat and, unseen to the thug, draws a bead on him from behind the canned vegetables. The customer's a good shot and has his choice of how he will aim at the guy. He could shoot the gun from his hand. He could fire in the air and tell him to drop it. He could do lots of things. But instead, the customer thinks, "I have always wanted to kill a man just to watch him die and this worthless piece of crap is gonna die." So he shoots the bandit in the head.It makes no sense to go into detail about the unknowable intentions of the shooter in this scenario if you are also going to conclude in advance that the shooting itself is "unnecessary." The shooter's reasoning is problematic, but his "use of force" is not therefore disproportionate or inordinate. A thug who pulls a gun "on" someone forces everyone involved to enter a world in which intentions are irrelevant in light of the time and risk involved. By drawing his gun, the thug is acting as if he intended to kill. He has effectively already begun the act of killing. There is no time to determine intentions because the gun is out and the finger is on the trigger--even an accidental twitch could end a life. The difference between intending and doing, with respect to the time of their happening, is negligible. A bunch of people holding guns to each others' heads only happens in the movies for a reason -- the ending is pre-determined by writers.
The Church would argue that this was not a "proportional" or "ordinate" use of force. Before the civil law, your customer would undoubtedly be acquitted as "killing in self defense." But morally, what he did was closer to murder, than self-defense, because it was unnecessary to kill the perp and he shot, not to defend himself, but because he proactively, rather than as a last resort, wanted to take human life.
So the idea is never that you "get" to kill. It is that you "have" to unfortunately kill and that, if you could possibly avoid it, you should. If we had phasers instead of guns, the Church was say that they had to be set to "stun."
Shea goes on:
So Thomas grants that those who are forced to kill in battle are doing so, not because they want to kill, but because they are merely trying to stop an aggressor. This is why, once the aggressor is stopped by an injury, you don't "get" to finish him off with a bullet to the brain and are, in fact, obligated to care for his injuries if he is, for instance, taken prisoner. The intention is not to kill, but to stop him.That depends on what "aggressor" means. If it means that every soldier in the other army is an aggressor by simply being within the horizon of your vision during a battle, then it makes some sense. If it means that you cannot morally "stop" an enemy soldier unless he is at that very moment obviously trying to kill you, then it's ridiculous.