The Sixth Commandment

Exodus 20:13 "You shall not murder.
 

Question:

Is it okay to kill when it's for someone you love, or for your country.
    Example: If you are in the Army and you get sent to war.
    Example: If your loved ones are killed and you know who did it.
 

Answer:
    You have asked some very difficult questions, and my heart wants to say, yes, it is OK. You see, I love my family and I love my country, and my heart tells me that I should protect both by whatever means necessary.
    But, I can't go by my heart, because the Bible says:

Jeremiah 17:9, 10  The heart is deceitful above all things and beyond cure. Who can understand it? "I the LORD search the heart and examine the mind, to reward a man according to his conduct, according to what his deeds deserve."

My heart can deceive me, but God won't. My heart may become blinded to the truth by the circumstances of life. But the Bible will always show me the truth. And the Bible says:
    Exodus 20:13 (NIV) "You shall not murder.”
    The King James Version says quite simply. 
    Exodus 20:13 (KJV) “Thou shalt not kill.”
    Your second question is easier to answer than your first one because the Bible answers it directly: “If your loved ones are killed and you know who did it.”

Rom 12:19-21 (NIV) Do not take revenge, my friends, but leave room for God's wrath, for it is written: "It is mine to avenge; I will repay," says the Lord. On the contrary: "If your enemy is hungry, feed him; if he is thirsty, give him something to drink. In doing this, you will heap burning coals on his head." Do not be overcome by evil, but overcome evil with good.

To kill someone who killed one of your loved ones is to take upon yourself the responsibility for punishing that person for his sins. That seems completely logical to us. But God says, “No. That's My job.” God will take vengeance upon the guilty, and that person will pay full measure for his sin—unless he repents.
    If you know the one who has broken the law, then it is your duty to report it to the authorities and give them all the evidence you have implicating the guilty party.
    But to take the law into your own hands not only breaks the ten commandments and the laws of the state, but it puts you on the same moral level as the one who committed the first crime. Then you are no better than he—in fact worse: because you know better and can see the picture more clearly than the guilty person did.
    So don't take vengeance. Let God and the proper authorities do that. Treat the person kindly, and that will bore holes into his heart that only God can heal. Just remember, when Jesus was hanging on the cross, he didn't curse his murderers or call down judgments upon them. (He could have vaporized them all!) Instead:

Luke 23:34 (NIV) Jesus said, "Father, forgive them, for they do not know what they are doing."

Life is precious. Only God can give life; we cannot.

Now about your first question: “If you are in the Army and you get sent to war.”
    You may grieve when a member of your family is killed in a war, and you may feel anger at the one who did it—even though you don't know who he is. This kind of anger, during wartime, is usually vented toward the country from which the killer-soldier came, and causes us to want to kill other enemy soldiers out of revenge.
    Turn the coin over. When you, as a soldier, kill an enemy soldier, how do you think his family feels about you? They feel grief at the loss of their loved one and anger toward you for killing him. Although they probably will never know you by name or face, they will feel anger toward your country and its armed services.
     Look at what's happening in the Middle East. There is no end to the cycle of revenge.
    So the commandment fits both circumstances. When God says, “Thou shalt not kill?” He means exactly that.
    Instead of anger and killing, God calls us to love our neighbor as ourselves. (Lev 19:18; quoted by Jesus in a conversation with a gentleman in Judea, in the region beyond the Jordan River:

Matthew 19:16-19  Now a man came up to Jesus and asked, "Teacher, what good thing must I do to get eternal life?" "Why do you ask me about what is good?" Jesus replied. "There is only One who is good. If you want to enter life, obey the commandments." "Which ones?" the man inquired. Jesus replied, "'Do not murder, do not commit adultery, do not steal, do not give false testimony, honor your father and mother,' and 'love your neighbor as yourself.'"

And in the sermon on the mount Jesus told us how to do that:

Matthew 7:12  So in everything, do to others what you would have them do to you, for this sums up the Law and the Prophets.

We may question what Jesus said, because this just isn't logical to us, it just doesn't seem to fit our world. We live in a world where nations send out armies to kill the soldiers—and often the civilians—of other countries. We conveniently label them “the enemy,” and to our minds that makes killing them all right. And yes, the hard facts are that if we don't kill them, they just might kill us. And so we come down to the nitty-gritty: his life or mine? How sad that such choices ever have to be made.
    I have to say again: Life is precious. Only God can give life; we cannot.
    I never had to serve in a war (I'm 67 at the time of writing) because I was never drafted (that's the way they filled the services in my time). As I said before, I love my country. I fully expected that I would be drafted, and if I had been drafted, I would have served. I'm not a draft dodger. 
    But even though I was not called to serve, I had already decided that my service would be that of a non-combatant—I would not carry arms. Instead, I would serve as a medic where I would be out there, under fire, saving lives rather than taking them.
    Each person must make his own choice in this, as in every other matter dealing with our life-time relationship with God. I have known some fine Christians who chose to bear arms and fought in wartime. I am not their judge. That is something God will have to decide.
    Take this to the Lord in prayer. Open your Bible and read the ten commandments (Ex. 20). Read about the closing hours of Jesus’ life on earth—from the time of the last supper till He was laid in the tomb. (Matthew 26-28; Mark 14-16; Luke 22-24; John 13-19) Pray that God will reveal to you through Jesus what you should do. Claim the promise:
    James 1:5, 6  If any of you lacks wisdom, he should ask God, who gives generously to all without finding fault, and it will be given to him. But when he asks, he must believe and not doubt, . . .
    God will give you the wisdom to make the right decision. And when you've made it, face the decision like a man. Follow God at all costs—even at the loss of life itself. For God has promised eternal life to those who faithfully serve Him.
    One last word: As a Christian, you are in God's army first and foremost. God's Kingdom is founded on love—love for God and for your fellow men, and the Bible is your training manual. Utterly refuse to do anything that the Bible does not condone. Then, as a Christian soldier, serve your family and your country with all the dignity that God bestows upon His faithful children.