IT was something I hadn’t talked about. A lifetime ago, at the age of 19, I killed two dying men. It was 1952 and during the Malayan "Emergency" when I was a National Service lieutenant in 42 Commando Royal Marines. In the aftermath of an ambush, two of the enemy (Communist guerrillas at war with "British imperialism") lay horribly wounded on the ground.

One lacked part of his skull, brains spilling on the earth. The other had half his rib-cage shot away, heart and a lung hanging out. Unconscious but still moving. Without waiting to reflect, I shot them both dead. A Marine muttered: "You wouldn’t leave a dog like that."

What do you call that? Mercy killing, "putting them out of their misery"? Murder ? Last week, I found I had to talk about this, because in a few days the case of Sergeant Alexander Blackman, also of 42 Commando Royal Marines, comes up for appeal. In 2011, in Helmand, he shot and killed a mortally wounded Taliban.

For that, he was condemned and sentenced for murder. The stress of battle, possibly pity, the chance that the man was dead before he fired – all these factors suggested a defence for culpable homicide. But none was offered to the court.

A massive miscarriage of justice resulted. So I wrote about my own experience in an article published in one London paper and promptly pirated by another. I implored British courts to take into account all the conflicting instincts, the adrenalin surges of fear and hate, which compete in war with the Geneva Convention rules.

Old soldiers and younger war-fighters may shake their heads. They know that both mercy killing and "taking out" inconvenient prisoners happen all the time in big and small wars. But civilians won’t make the effort to understand why, so "it’s better to keep quiet".

I couldn’t keep quiet for several reasons. One was the blatant unfairness of the Blackman case. Another was that the way it was presented defaced the name of the Commando in which I had served and which I still respect. I trusted the Marines’ guts and professionalism; to me, at 19, they were like the men who marched with Alexander of Macedon to the ends of the known world. But a third reason was that the killing of those two men had begun to upset me more and more.

For many years, I didn’t dwell on it. Other things about that fight seemed more memorable: the Chinese communist who threw away his life trying to save a comrade, the half-mad faces of our men as they fired, the Australian wife of the local rubber planter who arrived on the scene with her dogs. They licked the blood. She cried: "Why didn’t I bring my camera?"

In middle age, I thought about the two young men more often. But without doubts. Yes, they were helpless and I killed them both, but it was obviously the best thing for them. Wasn’t it? So that's me off the hook: uneasiness sorted.

But in old age my feelings have changed. For some years, those deaths have haunted me. One of the boys was the son of a Chinese bicycle dealer in a village in Pahang, far away in the tranquil north-east of Malaya. I picture that white shop house with its red roof, the scents of dried fish and coconut oil and charcoal hearths which must have been his scents of home. Did he have brothers and sisters? Does anyone remember him?

The two were at the start of their time on earth. But so was I when I ended their lives. As I approach the last tract of my own time here, I am awed by the sheer privilege of existing in this beautiful, restless, endlessly-astonishing profusion. How much they lost becomes clear to me every day.

That said, talking about this now is not primarily to do myself a favour – to "get it off my chest" or "come to terms" with it. If the injustice done to Sergeant Blackman had not come to a new point of decision – the appeal is on February 7 – I would probably have kept silent. Staying quiet about bad things, whether committed or simply experienced, is not fashionable in a society which never tires of flushing its worries in public. But I have learned to respect that silence, mostly by listening to elderly Polish and Jewish survivors of Hitlerism and Stalinism.

I have seen things which no human being should see, or even know that they are possible. I do know, so the only thing I can do is to make sure that others – my children, above all – are not poisoned by this knowledge.

I will keep silent, so that the evil stops with me.

The trouble with that line, noble as it may be, is that it allows "ordinary people" to give up trying to understand war. I am sick of "nobody who hasn’t been in combat can imagine what it’s like". Yes, they can – with a bit of effort. The evidence is in every bookshop and cinema, and there can be few people – especially in Scotland – who don’t know a man or woman with that experience, whether in Normandy or Afghanistan. All it takes is a genuine wish to know. Don’t go for an excuse like "war is a foreign country; they do things differently there". Yes, they do, but if you learn their foreign language, you will understand how.

Be sceptical, too, about the tabloid habit of calling everyone on active service a "hero". It was simpler in the Second World War, when millions of civilians shared the impact. They knew that in the Blitz, some were brilliantly brave and unselfish, but that others were traumatised into cowards and thieves.

Most ex-servicemen who have actually fought get through their lives without much self-reproach, if any. (That’s not the same as saying that they don’t often suffer every degree of traumatic stress.) They did their job for Queen, regiment and country; the enemy dead are anonymous, and compassion for them is limited. As Sergeant Blackman said after shooting the Taliban: "He’d do the same to me."

Looking back, I ask myself again: did I do the right thing? Yes, I did. What else could I do – leave them there in agony? So I did the right thing, but it was also the wrong thing. I don’t mean that it was murder, although the obsolete law-books which sent Sergeant Blackman to jail give it that name. But to kill the helpless is a wrong in itself, however valid the reasons and excuses and justifications are. I think that Adomnán, the great Abbot of Iona who succeeded St Columba, understood this foul riddle which is inherent in warfare when he wrote his Law Of The Innocents. Addressed to the bloodthirsty kinglets of Ireland, this was none the less Europe’s first attempt to draft an agreement limiting war: men will kill but women and children must be spared.

Many people, not only the religious, believe in absolution. Sin is confessed and then simply wiped out, not merely forgiven because of "extenuating circumstances". In old age, that begins to look rather different. The wrong of taking a life somehow separates from all the sound justifications, and from all efforts made afterwards to compensate for it.

Two illustrations, both German. Kurt Gerstein was an SS officer: he ordered the gas for Auschwitz, and in 1942 witnessed the gassing of 3,000 Jews at the Belzec death camp in Poland. A few days later, he met a Swedish diplomat on a night train and begged him to tell the world what was happening. He went on to spread the news of the Holocaust to Resistance movements and the Allies. The Nazis never caught him. But in 1945, when the war was over, he killed himself.

Three centuries earlier, the priest Friedrich Spee wrote a tremendous book exposing the torture and burning of innocent women accused of witchcraft. He knew they were innocent because he himself had served for years as "confessor" in the torture chambers and stood by as the women burned. (Donald Trump should read what he wrote about the value of evidence extracted by torture).

His book, Cautio Criminalis, helped to halt the whole witch-hunt in Catholic Germany. But he never felt that it absolved him. Others did. Yet Spee took the guilt with him to the grave.

Extreme examples? Many will say: "If we hadn’t killed Germans, Hitler would have killed us. Where’s the problem? Get over it!" Somebody else might say: "If I try to escort these Taliban (or Iraqi or Argentinian) prisoners back across that field of fire, I risk losing most of my own guys. So – it’s not pretty and I’m sorry for their mums, but it’s a no-brainer." And a friend on this paper asked me: "Why was it OK for you to kill those men as they came out of the jungle, but wrong to kill them when they lay mortally wounded?"

I stand by what I did. How could I have done anything else and lived with myself? But common-sense arguments don’t blot out death. Would I do it again? No, because now I couldn’t take a life on impulse.

War itself is the riddle. It's unbelievable that we still do it. Sometimes it’s "right", but it always involves wrong. War is the moral maze with an entrance but no exit – and no pretty statue to be found in the middle.