Actually, both words are acceptable, but murder is listed first in BAGD or Bauer. In fact, Bauer even lists Romans 13:9 as an example for the word, foneuseis or φονεύσεις, which is the Future Indicative Active, 2nd person, singular.
Paul actually quotes from the Septuagint, not the Hebrew, because the commandments are not listed in the same order as the Hebrew. As far as the difference between kill and murder, I think essentially they are almost the same thing! At least as far as people are concerned. Because if you kill a person, you are murdering them, right? However, in defense of the KJV, perhaps the meaning has changed over 400 years. Maybe kill implied murder? But against the KJV, by using the larger meaning of kill, this has the implication that even the state kills a person as punishment for a crime it is wrong. And killing would also encompass accidental death, unintentional manslaughter because that would be killing and therefore against God's law.
Lots of implications in these words. But as a translator, I would have to go with murder, not kill, at least in modern English.