Ratsach (רָצַח) means to murder or to kill unlawfully. It appears about 47 times in the OT and is the specific verb used in the sixth commandment: "You shall not murder [ratsach]" (Exod. 20:13, Deut. 5:17). It is carefully distinguished from other Hebrew killing words:
Harag (H2026) = general killing/slaying; muth (H4191) = to die or be put to death; nakah (H5221) = to strike/smite; shafak (H8210) = to shed (blood). Ratsach specifically denotes premeditated murder, manslaughter, or killing that is morally culpable — as opposed to warfare or lawful capital punishment.
The sixth commandment's prohibition against ratsach is grounded in the imago Dei — human beings bear the image of God (Gen. 1:26-27, 9:6), and therefore to murder a human is to strike at God's image in the world. Human life has intrinsic dignity because of whose image it bears.
Jesus radicalized the prohibition in the Sermon on the Mount: "You have heard that it was said to those of old, 'You shall not murder,' and whoever murders will be liable to judgment. But I say to you that everyone who is angry with his brother will be liable to judgment" (Matt. 5:21-22). The commandment prohibits not merely the act but the heart disposition — hatred, contempt, and unjust anger toward the image-bearer. 1 John 3:15: "Everyone who hates his brother is a murderer."