There is an article written by a professor that discusses the mistranslation of the Bible and says,
Indeed, "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. (http://www.ucalgary.ca/~elsegal/Shokel/001102_ThouShaltNotMurder.html)
Raised as a Roman Catholic (that summarily dismissed it from as early back as I remember), the roots of my own misquote of the 6th Commandment is understandable. That is at the root of all organized religions: The followers must accept unconditionally what they are told and take it to represent meaning for themselves. Psychologists would call this an external locus of control. Sooner or later, some little tidbit like this will surface that threatens the stability of the bases for morality and values, i.e., cognitive dissonance. There are many other 'mistranslations' of the original text of the Bible, so it is no surprise that there is yet another.
Myself, I have to say that it is against my morals and values to take a life unnecessarily. This puts me in the very uncomfortable and very immoral position of judge. I can and will only judge for myself, my own actions; not another's. This is 'co-measurement'. I choose to live morally and aware. This includes not standing in judgement of others. Therefore, I lean toward "thou shalt not kill" more so than "thou shalt not murder".
There are many other facets to this discussion that I won't go into now.