Objection 1. It would seem unlawful to kill any living thing. For the Apostle says (Romans 13:2): "They that resist the ordinance of God purchase to themselves damnation [Vulgate: '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 Psalm 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.
Objection 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.
Objection 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 (Exodus 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."
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. On fact this is in keeping with the commandment of God Himself: for it is written (Genesis 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 (Genesis 9:3): "Everything that moveth and liveth shall be meat to you."
Reply to Objection 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 to Objection 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 to Objection 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.
Source: Summa Theologica (New Advent)