Catechism of the Catholic Church §2321 — excerpt
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.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.