Let’s get one thing straight: it’s not “Thou shalt not kill” (which would be “lo taharog” in Hebrew), it’s “Thou shalt not murder” (which is “lo tirtzach” in the Torah). There’s a huge difference between the two. Sometimes circumstances force us to kill, for example in self-defense, in battle, or when executing a convicted felon. But we may not murder, which is to wantonly slaughter innocents.
We were put on this world to build the world and to settle it. To multiply and fill the world is the very first commandment. To kill innocents goes completely counter to that ideal. Everyone enjoys the right to live, though certain people forfeit that right through their destructive deeds. The Sefer HaChinuch cites the Talmud in Baba Metzia 83b that executing criminals convicted of capital crimes is analogous to weeding one’s vineyard in that it actually fosters the growth of society rather than impeding it.
Human life is so precious that even closing the eyes of a dying person, hastening death by mere seconds, makes one a full-fledged murderer and liable to the same punishment as one who killed a healthy individual (Semachot 1:4).
Murder is one of the three mitzvos for which one must be prepared to give up his own life rather than violate it. (The others are idolatry and certain forbidden sexual relations.) These mitzvos are called “yehareg v’al ya’avor,” meaning “allow oneself to perish rather than violate.” The rationale in the case of murder is that we have no way of determining whose life is more important. Therefore, one may not sacrifice another to save himself (see Talmud Sanhedrin 74a).
The prohibition against murder applies to both men and women in all times and places. This is #289 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #32 of the 194 negative mitzvos that can be fulfilled today in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar. In the Talmud it is discussed in the ninth chapter of Sanhedrin and in the second chapter of Makkos; in Shulchan Aruch, it is found in Choshen Mishpat 425.