Here’s what Fr Heribert Jone in Moral Theology says. Fr Jone is an “approved author” and so his work is supposed to have some authority. This is no. 215 (p. 140):
II An unjust aggressor may be killed if the following conditions are verified:
- The goods to be defended must be of great value.
Such goods are: life, integrity of one’s members, chastity, temporal goods of great value . . . One may defend the life and possessions of others even as he may defend his own.
So defending the life of an unborn child would meet this criterion.
2. The aggression must be actual and unjust . . .
a) Actual aggression is an imminent or practically present assault which cannot be evaded.
Self-defense is, therefore, allowed not only when actually covered by the bandit’s gun, but when the assailant reaches for his knife or revolver, sets his dog on one, summons his accomplice; not however, if there is question merely of threatened or anticipated attack . . .
I think is the key criterion to consider. It seems that killing an abortionist while he is just having breakfast at home would not be allowed since the aggression is not “imminent” or “practically present assault.” I guess it would be allowed if the abortionist were grabbing the tools with which he is about to kill a baby. We would probably never be in a circumstance where we are in the proximity of an abortionist who is grabbing his abortion tools.
b) Unjust aggression is had when the assault is at least materially unwarranted.
Therefore, one may also kill an insane or intoxicated person in self-defense.
So even if the abortionist doesn’t know that what he is doing is wrong, his action still meets this criterion.
3. The defense must be moderate, i.e., the assailant must not be injured more than is absolutely necessary to insure self-protection known as “moderamen inculpatae tutelae” or moderation of blameless defense.
. . . Killing is unlawful if the assailant can be rendered harmless by wounding him . . .
So if injuring or otherwise incapacitating an abortionist prevents the abortion one is trying to stop then one must limit one’s self to that means. One could argue that killing an abortionist stops not just one abortion but a whole lot of abortions, but then we run into criterion 2a which specifies that the assault must be “imminent” or “practically present.”
N.B. There is generally no obligation to defend one’s self when this is possible only by killing the assailant.
. . . There may be a duty of charity to defend others (wife, children, parents, relatives) against an unjust aggressor. Officers of the law, etc., may be obliged to defend others against unjust aggression by reason of their office.
It seems that if the person is your child then that may affect whether you have a duty of charity to defend him or her. It also seems that if the defense involves killing the assailant then generally those no obligation to kill him. I’m not sure why that is, but it makes sense to me. Unfortunately this book doesn’t explain much why things are so.