Therefore this act, since one’s
intention is to save one’s own life, is not unlawful, seeing that it is
natural to everything to keep itself in “being,” as far as possible. And yet, though proceeding from a
good intention, an act may be rendered unlawful, if it be out of proportion to the end. Wherefore 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 own life than of another’s. But as it is unlawful to take a man’s life, except for the public authority acting for the common
good, as stated above (
Article 3), it is not lawful for a man to intend killing a man in self-defense,
except for such as have public authority, who while intending to kill a man in self-defense, refer this to the public good, as in the case of a soldier fighting against the foe, and in the minister of the judge struggling with robbers, although even these
sin if they be moved by private animosity.