I am Polish. In my native language ‘murder’ and ‘kill’ are almost synonymous. Naturally, I speak English and I know that ‘kill’ is a lot weaker compared than ‘murder’.
Nevertheless, Ten Commandments are translated into contemporary Polish with a negative form “Don’t...”, since the form “You shall not...”, we have in medieval translations of the Bible, is very archaic in the Polish language nowadays.
While the English word ‘kill’ may be used to objects too, it’s just beyond comprehension to use this word in Polish that way. In the Polish language, “kill” means to deprive a LIVING BEING of its/his/her life. When applied to animals, it sounds weak, but as to humans, it sounds strong enough, almost like ‘murder’.
Personally, I thing the word “murder” is the proper one, but using the word ‘kill’ is a question of a long-aged Polish tradition.
The word ‘kill’ may be used to animals, as well. However, everybody instinctivelly knows, which is obvious, God refers to human beings only, not animals. What’s more, He does it the way the English word ‘murder’ sounds perfectly enough to express His real intentions.