Thou shalt not kill
The previous commandments deal with authority: the authority of God, our parents and civil leadership. The rest of the Commandments turn our attention to our neighbor and how we are to treat them. The Fifth Commandment requires Christians to stand on the side of life. It clearly prohibits senseless violence and murder, but what does it say about warfare, termination of life, about military service? The answers to these questions are not so clear cut. The Fifth Commandment does demand that all effort be made to preserve and protect all life. As Christians, we are to build up and care for one another in all circumstances.
Q. What does this mean?
A. We are to fear and love God, so that we will neither harm nor hurt our neighbor's body, but help him and care for him in all his physical needs.
From Luther’s Large Catechism
182] Now this commandment is easy enough, and has been often treated, because we hear it annually in the Gospel of St. Matthew 5:21ff, where Christ Himself explains and sums it up, namely, that we must not kill, neither with hand, heart, mouth, signs, gestures, help, nor counsel. Therefore it is here forbidden to everyone to be angry, except those (as we said) who are in the place of God, that is, parents and the government. For it is proper for God and for everyone who is in a divine estate to be angry, to reprove and punish, namely, on account of those very persons who transgress this and the other commandments.
183] But the cause and need of this commandment is that God well knows that the world is evil, and that this life has much unhappiness; therefore He has placed this and the other commandments between the good and the evil. Now, as there are many assaults upon all commandments, so it happens also in this commandment that we must live among many people who do us harm, so that we have cause to be hostile to them.
186] Thus this commandment aims at this, that no one offend his neighbor on account of any evil deed, even though he have fully deserved it. For where murder is forbidden, all cause also is forbidden whence murder may originate. For many a one, although he does not kill, yet curses and utters a wish, which would stop a person from running far if it were to strike him in the neck [makes imprecations, which if fulfilled with respect to any one, he would not live long].187] Now, since this inheres in every one by nature and it is a common practise that no one is willing to suffer at the hands of another, God wishes to remove the root and source by which the heart is embittered against our neighbor, and to accustom us ever to keep in view this commandment, always to contemplate ourselves in it as in a mirror, to regard the will of God, and with hearty confidence and invocation of His name to commit to Him the wrong which we suffer. Thus we shall suffer our enemies to rage and be angry, doing what they can, and we learn to calm our wrath, and to have a patient, gentle heart, especially toward those who give us cause to be angry, that is, our enemies.
188] Therefore the entire sum of what it means not to kill is to be impressed most explicitly upon the simple-minded. In the first place, that we harm no one, first, with our hand or by deed. Then, that we do not employ our tongue to instigate or counsel thereto. Further, that we neither use nor assent to any kind of means or methods whereby any one may be injured. And finally, that the heart be not ill disposed toward any one, nor from anger and hatred wish him ill, so that body and soul may be innocent in regard to everyone, but especially those who wish you evil or inflict such upon you. For to do evil to one who wishes and does you good is not human, but diabolical.
189] Secondly, under this commandment not only he is guilty who does evil to his neighbor, but he also who can do him good, prevent, resist evil, defend and save him, so that no bodily harm or hurt happen to him, and yet does not do it.190] If, therefore, you send away one that is naked when you could clothe him, you have caused him to freeze to death; if you see one suffer hunger and do not give him food, you have caused him to starve. So also, if you see any one innocently sentenced to death or in like distress, and do not save him, although you know ways and means to do so, you have killed him. And it will not avail you to make the pretext that you did not afford any help, counsel, or aid thereto, for you have withheld your love from him and deprived him of the benefit whereby his life would have been saved.
Large Catechism Text in the public domain. http://bookofconcord.org/lc-3-tencommandments.php