On Facebook over the past couple weeks, one of my posts about the "Top Ten Conservative Movies" has led to a long, rambling debate about just war and the ethics of self-defense, which you can read here. The latter has been particularly interesting. In the course of the discussion, Exodus 22:2 was brought up, as proof that it was morally right to kill an intruder who might be threatening your life and that of your family's. I countered with the suggestion that there is a distinction between what is justly lawful and what is righteous. This distinction particularly applies in view of the Old Testament/New Testament progression, in which Christ tells us that what was permitted because of the people's hardness of heart in the Old Testament should be transcended by faithful believers in the New. Given that civil authority in this present ages is a holdover from the past age, in power only insofar as Christ's kingdom has not yet fully replaced it, could it be that killing in self-defense ought to be permitted by the law for the sake of order, but ought not to be practiced by Christians?
This is certainly what Augustine thought. Because they ought not to fight to defend what is not theirs to keep (including their lives and their property), the attempt to do so was the result of a sinful lust, which desired too much what was not worth such desire. There was no just way for someone to defend themselves or their property to the point of shedding blood. Nevertheless, because it would be better for civil order that a wicked attacker should die rather than an innocent victim, the law ought not to prosecute those who kill in self-defense. Here's an excerpt from De Libero Arbitrio:
"Surely, I think that a law is quite safe from this accusation [of injustice] if it permits the people it rules to do lesser evils so as to avoid greater ones. It is much better that the man who plots against another's life be killed than the man who is defending his life. It is also much worse for an innocent person to be violated than for the assailant to be killed by the person whom he tried to attack....But even though the law is blameless, I do not understand how these men can be, when the law does not force them to kill, but leaves it to their power. They are free not to kill anyone for those things which they can lose against their will and which they ought not therefore to love.
"Concerning life, perhaps there is some question whether or not it can be taken away in any way from the soul when body is slain. But if life can be taken away, then it is to be despised. If life cannot be taken away, then there is nothing to fear."
Dr. O'Donovan tells me that this was the standard Christian teaching until the introduction of "rights" language by the canonists in the 13th-century.