St. Thomas Aquinas, known as the Angelic Doctor in the Catholic Church, tackled the question of the legitimacy of self-defense in his Summa Theologica. He did this in a discussion of the legitimacy of using force sufficient to kill an aggressor if the situation necessitated that.
St. Thomas recalled that, according to Exodus 22:2, “if a thief be found breaking into a house or undermining it, and be wounded so as to die, he that slew him shall not be guilty of blood.” St. Thomas then commented that, “it is much more lawful to defend one’s life than one’s house. Therefore neither is a man guilty of murder if he kills another in defense of his own life.”
According to St. Thomas Aquinas, “if a man, in self-defense, uses more than necessary violence, it will be unlawful: whereas if he repel force with moderation his defense will be lawful, because according to the jurists (Cap. Significasti. De Homicid. volunt. vel casual.), ‘it is lawful to repel force by force, provided one does not exceed the limits of a blameless defense.’ Nor is it necessary for salvation that a man omit the act of moderate self-defense in order to avoid killing the other man, since one is bound to take more care of one’s life than of another’s.”