The putative author of the proscription against killing was neither of those. "He" (Yahweh) is alternately "they" (Elohim). That Jewish monotheism evolved from an earlier polytheism is evident from the statement
In the beginning the gods created the heavens and the Earth. I retained the King James idiom in this. A better translation is "Long ago the gods made the sky and the ground." But regardless how you parse it, their
Elohim is "gods", not "God".
The evolution into monotheism was a seminal event in the founding of modern Judeo-Christian and Islamic tradition. It was enhanced in the early Christian era by the splintering of God into a Trinity. It's no coincidence that the Caesars of Rome had recently invented the notion of a
Triumvirate. This kept alive the appeal to the more ubiquitous polytheism of the Levant. After all, three Gods (or a three-headed God) are better than one.
Other evidence of the polytheism of the proto-Jewish cult, and continuing in other forms thereafter, includes a vase with an inscription to "Yahweh and his
Asherah", as well as of the widespread worship of Asherah as the Queen or Heaven, in the form of thousands of figurines uncovered in excavations of ancient sites in modern Israel.
Syncretism between Greek, Persian, Egyptian, Assyrian, Babylonian and (earlier) Ugaritic and (later) Roman traditions brought to Judaism a reinvention of God to include a Son of God (an idea borrowed from most all of those cults) who drank from the bitter cup (the Greek Socrates, and the passion of his execution for belief in one God) at the hour of his death, surround by his 12 followers (Persian-Roman Mithra, the Sun, followed by the zodiac. They apparently believed
Mithra was crucified after a last supper with the 12, and commemorated this with ritual meals, evidenced by sites left behind in the Roman catacombs, which are associated with the origins of the Mass. The death of the Sun at the winter solstice only to rise again on the 3rd day (Dec 25) is probably borrowed from Mithraism. It's no wonder then, that the Christian version also shifted the relationship of the Queen of Heaven away from the less relevant role as wife and co-ruler to mother, giving them the fantastic problem in wedding Yahweh to Mary metaphysically, in order to create Jesus. This may also tell us that they authors of this tale were aware of the Immaculate Conception of gods as far away as
India.
There never was a One and Only, regardless of how popular the notion is - and has been for centuries. That's just the innovation of syncretism, a consequence of the captivity of Isrealites in Babylon, the Greek incursion into Persia and the Levant, and the experience under the Roman triumvirate, especially the ca. 69 AD destruction of the Temple and the mass crucifixions of Zealots. Obviously the One and Only was a turning point in the evolution of Judaism, but it's a relatively late innovation. This is why there are two versions of the Israelite creation myth in Genesis, one by the gods
Elohim, and a second one by the singular Yahweh. The story was being amended, or preserved for both cults, when apparently a subsequent rule against further alterations locked it in place.
"Though shalt not kill" is preceded in the history of law by the familiar "an eye for and eye" Code of Hammurabi, the demigod king of ancient Babylon, another syncretic import, as well as the story of a man named Utnapishtim who was saved from the flood by
being warned to build a boat and stock it with animals, a saga known as the Gilgamesh Epic. The code of Hammurabi is practically as detailed as a modern criminal code, reserving a particular punishment for each of hundreds of different injustices, although they were quite cruel.
And the list goes on. Among all of the world's religions, Judaism, and the cults that stemmed from it, carries one of the most diversified collection of ideas melded from the many cultures springing up all around them for many centuries. Canaanites were closely related to Phoenicians, world travelers and merchants, who obviously had a chance to learn a lot of the myths and lore of the entire ancient wold of the Levant and many a distant Mediterranean harbor.
One of Many is a better characterization than One and Only.