Is the commandment "You shall not kill" or "You shall not murder"?
The Hebrew word in Exodus 20:13 and Deuteronomy 5:17 is ratsach. It is not the commonly used words for killing, which are haragh (to slay) and muth (to die). Ratsach is most frequently used in Numbers 35 where rules are given to determine if the death of a person was purposeful or accidental. Thus we conclude that ratsach refers to the unauthorized taking of a human life, whether one committed a murder or was accused of murder. The only time ratsach is used in an approved way is in Numbers 35:30 where it states the murderer (ratsach) is to be put to death (ratsach). It is this one usage was to emphasize that the murderer would receive what was due. By the way, the word ratsach is never used in connection with the taking of the life of an animal.
The Greek word in Matthew 19:18 and Romans 13:9 is phoneuo. It means to commit murder. This was the Greek word the Septuagint writers chose to translate the Hebrew word ratsach. It is consistently used to refer to the forbidden act of taking a human life.