The two versions considered authoritative by the Church are the Vulgate and the Septuagint. The Septuagint has οὐ φονεύσεις for Ex: 20:15 ("thou shalt not kill", or "thou shalt do no murder"). φονεύω means to murder, or kill. The Vulgate says non occides. occido is a verb meaning to kill, cut up, slaughter (its root is ob, "towards", + caedere, "to cut"). The Douay-Rheims translates "thou shalt not kill", though Ronald Knox translates it "Thou shalt do no murder".  The New Jerusalem translates it "you shall not kill". I wonder if the two of you are actually disagreeing about what the difference between murder and killing is?