Thie question is far more hypothetical than you realize. After all, we know that God values human life even more than we do, so He isn't going to tell us to kill anyone. (Except under exceptional circumstances, eg Abraham, but again, Abraham had previously spoken directly to God, which is more than any of us can say.) I can't say what I would do if God told me to kill someone, for the simple reason that I can't think of any circumstances when He would. After all, if He needs someone dead, He has plenty of ways of doing it himself. And I see nothing wrong with obeying God's moral standards rather than my own - I understand imperfectly, but He with his knowledge understands all.
@Mathilda - When Mormons speak of salvation, there are 3 or 4 different things we mean.
In the broadest sense, everyone who has ever lived will be saved from physical death, ie will have a physical body in the next life and will be resurrected.
In the next sense, all but a very small group (those who, after having a sure knowledge of God, deny him - you pretty much have to be a prophet before you can commit what we call the unpardonable sin) will be saved from complete spiritual death, or living completely outside the presence of God.
In a more specific sense, those who have obeyed the commandments and accepted Christ will live with God, and be able to become like He is, which is usually what we mean by salvation.
It is because of Christ that we are saved from physical death, ie we can have a physical body after this life, and we can be saved from our sins if we accept him (which also means doing what he said to do). Of course, those who couldn't accept him in their earth life (like, you know, the majority of the human race, who never heard of him) will be able to after death.
We don't believe in the Trinity in the same sense as mainstream Christianity - we believe that Jesus is a separate and distinct being from God the Father. However, we do believe that he is and always has been divine, that he has a relationship to God that none of us has, and was chosen from the beginning to be the Savior of mankind. He is also fully human - when he was born, he became subject to both the physical infirmities and moral temptations that afflict humanity. Since we believe that humanity has a divine potential, saying that someone is both human and divine isn't a contradiction.