Thou shalt not kill. Exodus 20:13
"One mistranslation is Matthew 5:21, where most English version of the Bible read, "You shall not kill." This is a quotation of Exodus 20:13. The Hebrew word used here is "murder" (ratzach), and not kill (harag).
In Hebrew there is a clear distinction between those two words. The first (ratzach) means premeditated murder, while the second (harag) encompasses everything from justifiable homicide, manslaughter and accidental killing, to taking the life of an enemy soldier in war. The commandment very precisely prohibits murder, but not the taking of a life in defense of oneself or others.
It is difficult to explain how English translators made this mistake since the Greek language also has separate words for "murder" and "kill," and it is the Greek word for "murder" (not "kill") which is used in Matthew 5:21. Even with no knowledge of Hebrew, the English translators of the New Testament should here have correctly translated "murder," and not "kill."'
(Bivin, David, and Roy Blizzard, Jr. 1994. Understanding the Difficult Words of Jesus: New Insights from a Hebraic Perspective. Shippensburg, PA: Destiny Image Publishers., pgs 69-70)