(5) Whether it is lawful to kill oneself?
(6) Whether it is lawful to kill a just man?
(7) Whether it is lawful to kill a man in self-defense?
(8) Whether accidental homicide is a mortal sin? _______________________
FIRST ARTICLE [II-II, Q. 64, Art. 1]
Whether It Is Unlawful to Kill Any Living Thing?
Objection 1: It would seem unlawful to kill any living thing. For the Apostle says (Rom. 13:2): "They that resist the ordinance of God purchase to themselves damnation [*Vulg.: 'He that resisteth the power, resisteth the ordinance of God: and they that resist, purchase themselves damnation.']." Now Divine providence has ordained that all living things should be preserved, according to Ps. 146:8, 9, "Who maketh grass to grow on the mountains . . . Who giveth to beasts their food." Therefore it seems unlawful to take the life of any living thing.
Obj. 2: Further, murder is a sin because it deprives a man of life. Now life is common to all animals and plants. Hence for the same reason it is apparently a sin to slay dumb animals and plants.
Obj. 3: Further, in the Divine law a special punishment is not appointed save for a sin. Now a special punishment had to be inflicted, according to the Divine law, on one who killed another man's ox or sheep (Ex. 22:1). Therefore the slaying of dumb animals is a sin.
On the contrary, Augustine says (De Civ. Dei i, 20): "When we hear it said, 'Thou shalt not kill,' we do not take it as referring to trees, for they have no sense, nor to irrational animals, because they have no fellowship with us. Hence it follows that the words, 'Thou shalt not kill' refer to the killing of a man."