Charles Ellicott Commentary Exodus 20:13

"Thou shalt not kill." — Exodus 20:13 (ASV)

You shall not kill. —From the particular duties owed by children to their parents, the Divine Legislator proceeded to establish those general duties that people owe to their fellow human beings. And the first of these is respecting their life. The security of life is the primary object of government; and it has been well said that people originally coalesced into States for self-preservation (Aristotle, Politics 1.1). All written codes forbid murder; and in communities that are without written codes, an unwritten law condemns it. When God set a mark upon Cain (Genesis 4:15), He thereby marked His abhorrence of the murderer. The “seven precepts of Noah” included one that distinctly forbade the taking of human life (Genesis 9:6).

In all countries and among all peoples, a natural instinct or an unwritten tradition placed murder among the worst of crimes and made its penalty death. The Mosaic legislation on this point differed from others principally in the care it took to distinguish between actual murder, manslaughter (Exodus 21:13), death by misadventure (Numbers 35:23), and justifiable homicide (Exodus 22:2). Before, however, it made these distinctions, the great principle of the sanctity of human life needed to be broadly established; and so the law was given in the widest possible terms—You shall not kill. Exceptions were reserved until later.