Roman Catechism of the Council of Trent, 1566, Part III, 5, n. 4.
Thanks for the quote!
There is NO SUCH COMMANDMENT. And never was.
The tablets Moses brought down from Mt Sinai did not bear the Hebrew word for "kill;" not once.
This is the 6th Commandment, exactly as it appears in both of the oldest Decalogues in existence, 4QPaleoExodus m, and the Nash Papyrus. It reads (phonetically), "Lo tirtzach," which in Hebrew means "You shall do no murder."
Not "kill," "murder."
But murder was changed deliberately in several instances to killing, the most notable being St. Jerome's Vulgate Bible, possibly (and unfortunately) the most influential translation ever.
Jerome did it because his commission was to write a translation in the "common" or "vulgar" Latin, the Latin of the common people. And he didn't want they should be confused over the distinction, so HE CHANGED THE BIBLE.
It would be rather hypocritical of G-d to make a commandment about "not killing" when there are several other instances in the Old Testament when he instructs people to kill for him, up to and including several instances of genocide.