I Googled on this the last time the question came up and found a site which discussed it in detail. As I recall, there is one word in Hebrew which refers to killing of any kind (such as killing animals for food) and another word which refers only to killing other people in ways that are explicitly against the law and which is generally translated as "murder." That's the word used in the commandment.
However, the site also pointed out that there's at least one case in the Bible where the "murder" word is used to refer to killing someone who is a criminal renegade and therefore legally considered fair game, so there's clearly an ambiguity there.
I personally also have trouble believing that the Bible would present a set of commandments which are assumed to be prior to all human laws but one of which works out to "Thou shalt not kill unless your local legal codes say it's okay, and then anything goes." Nazi laws couldn't make genocide acceptable, and US laws can't make invading other countries and killing their civilian population acceptable either.
On edit: I remembered I'd saved the link and a quote from it:
http://www.ucalgary.ca/~elsegal/Shokel/001102_ThouShaltNotMurder.htmlIndeed, "kill" in English is an all-encompassing verb that covers the taking of life in all forms and for all classes of victims. That kind of generalization is expressed in Hebrew through the verb "harag." However, the verb that appears in the Torah's prohibition is a completely different one, "ratsah" which, it would seem, should be rendered "murder." This root refers only to criminal acts of killing.
<snip>
The fact remains, however, that even the Jewish translators were not unanimous in maintaining a consistent distinctions between the various Hebrew roots.
Don Isaac Abravanel and others noted that ratsah is employed in Numbers 35:27-30 both when dealing with an authorized case of blood vengeance, and with capital punishment -- neither of which falls under the legal category of murder.
In fact, some distinguished Jewish philosophers believed that "thou shalt not kill" is a perfectly accurate rendering of the sixth commandment. Maimonides, for example, wrote that all cases of killing human beings involve violations of the command, even if the violation happens to be overridden by other mitigating factors. It has been suggested that this tradition underlies the virtual elimination of capital punishment in Rabbinic law.
And if Maimonides believed that, who am I to argue? :shrug: