I answer that, There is no sin in using a thing for the purpose for which it is. Now the order of things is such that the imperfect are for the perfect, even as in the process of generation nature proceeds from imperfection to perfection. Hence it is that just as in the generation of a man there is first a living thing, then an animal, and lastly a man, so too things, like the plants, which merely have life, are all alike for animals, and all animals are for man. Wherefore it is not unlawful if man use plants for the good of animals, and animals for the good of man, as the Philosopher states (Polit. i, 3).
Now the most necessary use would seem to consist in the fact that animals use plants, and men use animals, for food, and this cannot be done unless these be deprived of life: wherefore it is lawful both to take life from plants for the use of animals, and from animals for the use of men. In fact this is in keeping with the commandment of God Himself: for it is written (Gen. 1:29, 30): "Behold I have given you every herb . . . and all trees . . . to be your meat, and to all beasts of the earth": and again (Gen. 9:3): "Everything that moveth and liveth shall be meat to you."
Reply Obj. 1: According to the Divine ordinance the life of animals
and plants is preserved not for themselves but for man. Hence, as
Augustine says (De Civ. Dei i, 20), "by a most just ordinance of the
Creator, both their life and their death are subject to our use."
Reply Obj. 2: Dumb animals and plants are devoid of the life of reason whereby to set themselves in motion; they are moved, as it were by another, by a kind of natural impulse, a sign of which is that they are naturally enslaved and accommodated to the uses of others.
Reply Obj. 3: He that kills another's ox, sins, not through killing the ox, but through injuring another man in his property. Wherefore this is not a species of the sin of murder but of the sin of theft or robbery. _______________________
SECOND ARTICLE [II-II, Q. 64, Art. 2]
Whether It Is Lawful to Kill Sinners?
Objection 1: It would seem unlawful to kill men who have sinned. For our Lord in the parable (Matt. 13) forbade the uprooting of the cockle which denotes wicked men according to a gloss. Now whatever is forbidden by God is a sin. Therefore it is a sin to kill a sinner.
Obj. 2: Further, human justice is conformed to Divine justice. Now according to Divine justice sinners are kept back for repentance, according to Ezech. 33:11, "I desire not the death of the wicked, but that the wicked turn from his way and live." Therefore it seems altogether unjust to kill sinners.
Obj. 3: Further, it is not lawful, for any good end whatever, to do that which is evil in itself, according to Augustine (Contra Mendac. vii) and the Philosopher (Ethic. ii, 6). Now to kill a man is evil in itself, since we are bound to have charity towards all men, and "we wish our friends to live and to exist," according to Ethic. ix, 4. Therefore it is nowise lawful to kill a man who has sinned.