Youcat commented through CCC – Question n. 380 – Part I.

YOUCAT Question n. 380 – Part I. Why is it permissible to tolerate the killing of another human being in the case of legitimate self-defense?

(Youcat answer) Someone who is actually attacking the lives of others may and must be stopped, if necessary by killing the attacker himself.

A deepening through CCC

(CCC 2263) The legitimate defense of persons and societies is not an exception to the prohibition against the murder of the innocent that constitutes intentional killing. "The act of self-defense can have a double effect: the preservation of one's own life; and the killing of the aggressor.... The one is intended, the other is not" (St. Thomas Aquinas, STh II-II, 64, 7,  corp. art.).      

Reflecting and meditating 

(Youcat comment) Legitimate defense against aggression is not only a right; for someone who bears the responsibility for the lives of others it can even become a duty. Nevertheless, legitimate defense must not employ wrong, inappropriately harsh methods.

(CCC Comment)

(CCC 2265) Legitimate defense can be not only a right but a grave duty for someone responsible for the lives of others. The defense of the common good requires that an unjust aggressor be rendered unable to cause harm. For this reason, those who legitimately hold authority also have the right to use arms to repel aggressors against the civil community entrusted to their responsibility. 

(This question: Why is it permissible to tolerate the killing of another human being in the case of legitimate self-defense? is continued)