The Old Testament always considered blood a sacred sign of life. 60 This teaching remains necessary for all time.
Catechism number 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."65