The sixth word says, “Thou shalt not kill.” What does that prohibit? Does it prohibit killing of all kinds – including killing animals? Does it prohibit all killing of human beings? Does it prohibit war or capital punishment?

Some translations use the word “murder” but that is too specific. Hebrew has a variety of words for killing. This one in the sixth word is used for the first time in the Bible.It is used frequently in Numbers 35, which lays out the institution of the cities of refuge, where manslayers can go in order to escape the avenger of blood. In this chapter, the verb often means “murder,” as in deliberate, intentional, premeditated killing (vv. 16, 17, 18, 19, 21, 25, 26, 27).

But the same verb is used frequently to describe what we call “manslaughter,” killing that is unintentional and not premeditated (v. 27; cf. Deuteronomy 4:42). Even the just vengeance of the avenger, which is basically a form of capital punishment, is described with the same word (v. 27). The death penalty is directly described by this verb (v. 30).
The word often means “murder” but it doesn’t always. It’s used for any peacetime killing of another human being. The Torah deals with different sorts of killing differently, but all killing is treated as a violation of the commandment that must be dealt with, either by the confinement of the manslayer to a city or refuge or by the execution of a murderer.

If the word means “killing” in this general sense, is it restricted to killing other human beings? What about animals?

In the beginning, Adam and Eve were arguably vegetarians, eating every plant yielding seed and tree yielding fruit (Genesis 1:29). After the flood, God give explicit permission to Noah to kill and eat animals, so long as he refrained from eating blood. Killing animals for food is legitimate.

But that’s not the end of the story. The Torah lays out various instructions about the care of animals. If you take eggs from a nest, you can’t take the mother and the eggs (Deuteronomy 22:6-7). Let the mother go, Moses says, so that your day may be prolonged.

If you find your enemy’s donkey wandering away, you should return it to him. If you find your enemy’s animal lying next to the road under a heavy burden, you should release it from its burden (Exodus 23:4-5). This is mainly about love for enemies, but it is also about care for animals.

Proverbs 12:10 says that a righteous man has regard for the life of his animals. The Sabbath applied not only to all the human beings in the household, but also animals. They too were supposed to receive the relief of a day off.

The Bible even gives us instruction about proper care of plants and the land. During war, Israel was forbidden to cut down fruit trees (Deuteronomy 20:19-20) to make siege works against a city. The tree is not a man, and so you shouldn’t make war against the tree. The land itself was supposed to have rest every seven years, just like the people and animals.

The Bible never forbids killing animals, and even requires is under some circumstances. But the Bible forbid gratuitous violence against animals and plants.