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

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

(Youcat answer - repeated) 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 2264) Love toward oneself remains a fundamental principle of morality. Therefore it is legitimate to insist on respect for one's own right to life. Someone who defends his life is not guilty of murder even if he is forced to deal his aggressor a lethal blow: If a man in self-defense uses more than necessary violence, it will be unlawful: whereas if he repels force with moderation, his defense will be lawful.... Nor is it necessary for salvation that a man omit the act of moderate self-defense to avoid killing the other man, since one is bound to take more care of one's own life than of another's (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 2321) The prohibition of murder does not abrogate the right to render an unjust aggressor unable to inflict harm. Legitimate defense is a grave duty for whoever is responsible for the lives of others or the common good.     

(The next question is: Why is the Church opposed to capital punishment?)