The Sixth Commandment of the Decalogue states “Thou shalt not kill.” Most people have understood this not to be purely pacifistic (as other parts of the Law, and the Old Testament itself, contradict this interpretation), but rather more like “You shall not murder,” which is definitely more distinct for our society (they would not have been confused).
So how can we keep the commandment not to murder? “Easy,” you might say, “Don’t kill anyone!” And that seems obvious enough. And it is true. However, that is not all there is. Why is it wrong to take a life impermissibly? Well, because, you are not permitted to do it! That is true but altogether unhelpful. Why is it that it is impermissible?
Jesus gives us a clue in the Sermon on the Mount, when he says if someone has hated his brother in his heart, he has violated the Sixth Commandment. This always struck me as austere. Is it really the same thing if I get unduly upset with my brother or if I stab him to death? This interpretation results in people saying things like, “If you hate your brother, you might as well go ahead and kill him!” This is wrong for two reasons. First, killing him would be an additional wrong, not the same instance of wrong, so at the very least there would be more sin in acting on the intention than merely the intention itself. Second, it genuinely seems worse to kill someone in action than in the mind. Note, I’m not saying it’s permissible.
But how then can we reconcile this? I think we reconcile our moral intuitions with the teaching of Jesus by understanding why the prohibition in the Sixth Commandment was made. Human beings are made in the image of God (Gen. 1:26, 2:7). There is no higher being that could ever be other than God; to the extent we reflect him, then, we are in that sense priceless. Thus, to kill a fellow human unjustly is to disregard the image of God in him, and thus is an affront to God himself (after all, what is rejected in image is a rejection of the one behind the image [to burn something in effigy is nothing else but to wish harm upon the one being caricatured; so to despise the image of God is nothing else but to despise God himself]). If we agree that God is the most holy, and he alone is to be worshipped and not supplanted, then the image of God in all human beings must be respected.
But this means that you cannot hate in your heart your fellow man. Why? Because that too fails to recognize the image of God in man (or worse, explicitly despises it) and thus does violence to the sacredness of God. Thus, whenever you hate your brother in your heart, you are despising the One who created him. So we can see a positive command in the prohibition: love the Lord your God with all your heart, soul, mind, and strength (sound familiar? This is linked to the First Commandment, and is called the greatest by Jesus!). Another implication: we are to respect all of our fellow human beings as created in the image of God, not just in word but in thought and deed. It can be expressed like this: Love your neighbor as yourself (sound familiar? Jesus taught this as the second-greatest commandment. Both of these can also be found in Deuteronomy 6). Thus, the Sixth Commandment is intricately involved with the issues of human life: worship toward God and love toward man.