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.”1
1 St. Thomas Aquinas, STh II-II, 64, 7, corp. art.