So its not murder if God tells you to do it? Try telling them that down at the police station XD
Short answer: no, it is not murder if God tells you to do it. God is the final arbiter of justice, not the police. Obviously, if you kill someone the police and the courts aren't likely to be convinced that you really heard from God, especially since many of them don't even believe in God or, if they do, most probably don't believe He speaks to people today in this way. Murder is, in essence, killing someone for your own personal reasons.
That's the simple answer. A proper understanding of killing from a Biblical standpoint would require considerably more study of history and context.