CS Lewis, "Mere Christianity":

Does loving your enemy mean not punishing him? No, for loving myself does not mean that I ought not to subject myself to punishment— even to death. If you had committed a murder, the right Christian thing to do would be to give yourself up to the police and be hanged. It is, therefore, in my opinion, perfectly right for a Christian judge to sentence a man to death or a Christian soldier to kill an enemy. I always have thought so, ever since I became a Christian, and long before the war, and I still think so now that we are at peace. It is no good quoting ‘Thou shalt not kill.’ There are two Greek words: the ordinary word to kill and the word to murder. And when Christ quotes that commandment He uses the murder one in all three accounts, Matthew, Mark, and Luke. And I am told there is the same distinction in Hebrew. All killing is not murder any more than all sexual intercourse is adultery. When soldiers came to St John the Baptist asking what to do, he never remotely suggested that they ought to leave the army: nor did Christ when He met a Roman sergeant-major— what they called a centurion. The idea of the knight— the Christian in arms for the defence of a good cause—is one of the great Christian ideas. War is a dreadful thing, and I can respect an honest pacifist, though I think he is entirely mistaken. What I cannot understand is this sort of semi-pacifism you get nowadays which gives people the idea that though you have to fight, you ought to do it with a long face and as if you were ashamed of it. It is that feeling that robs lots of magnificent young Christians in the Services of something they have a right to, something which is the natural accompaniment of courage— a kind of gaiety and wholeheartedness.

I have often thought to myself how it would have been if, when I served in the First World War, I and some young German had killed each other simultaneously and found ourselves together a moment after death. I cannot imagine that either of us would have felt any resentment or even any embarrassment. I think we might have laughed over it.

I imagine somebody will say, ‘Well, if one is allowed to condemn the enemy’s acts, and punish him, and kill him, what difference is left between Christian morality and the ordinary view?’ All the difference in the world. Remember, we Christians think man lives for ever. Therefore, what really matters is those little marks or twists on the central , inside part of the soul which are going to turn it, in the long run, into a heavenly or a hellish creature. We may kill if necessary, but we must not hate and enjoy hating. We may punish if necessary, but we must not enjoy it. In other words, something inside us, the feeling of resentment, the feeling that wants to get one’s own back, must be simply killed. I do not mean that anyone can decide this moment that he will never feel it any more. That is not how things happen . I mean that every time it bobs its head up, day after day, year after year, all our lives long, we must hit it on the head. It is hard work, but the attempt is not impossible. Even while we kill and punish we must try to feel about the enemy as we feel about ourselves— to wish that he were not bad, to hope that he may, in this world or another , be cured: in fact, to wish his good. That is what is meant in the Bible by loving him: wishing his good, not feeling fond of him nor saying he is nice when he is not.