Is murder allowed if God tells you to do it?

The Bible pretty clearly says not to murder ([Exodus 20:13](http://www.biblegateway.com/passage/?search=Exodus+20:13&version=NIV)) . But if we examine [Numbers 25](http://www.biblegateway.com/passage/?search=Numbers%2025&version=NIV) or [1Kings 18:40](http://www.biblegateway.com/passage/?search=1%20Kings%2018:40&version=NIV) , we see God (or more accurately, his prophets) telling us to kill people. Indeed, entire groups of people were murdered because of their mistaken beliefs. So, is it alright to murder if God or one of his prophets tells you to? I would think that this concept against murdering would be against all Christian doctrine. However, how does the mainstream Protestant groups (Baptist, Episcopalian, Methodist, Lutheran, etc.) reconcile these two concepts?