I'm getting a bit tired of the "deranged" soldier story. It was predictable, of course. The 38-year-old staff sergeant who massacred 16 Afghan civilians, including nine children, near Kandahar this week had no sooner returned to base than the defence experts and the think-tank boys and girls announced that he was "deranged". Not an evil, wicked, mindless terrorist – which he would be, of course, if he had been an Afghan, especially a Taliban – but merely a guy who went crazy.

This was the same nonsense used to describe the murderous US soldiers who ran amok in the Iraqi town of Haditha. It was the same word used about Israeli soldier Baruch Goldstein who massacred 25 Palestinians in Hebron – something I pointed out in this paper only hours before the staff sergeant became suddenly "deranged" in Kandahar province.

"Apparently deranged", "probably deranged", journalists announced, a soldier who "might have suffered some kind of breakdown" (The Guardian), a "rogue US soldier" (Financial Times) whose "rampage" (The New York Times) was "doubtless [sic] perpetrated in an act of madness" (Le Figaro). Really? Are we supposed to believe this stuff? Surely, if he was entirely deranged, our staff sergeant would have killed 16 of his fellow Americans. He would have slaughtered his mates and then set fire to their bodies. But, no, he didn't kill Americans. He chose to kill Afghans. There was a choice involved. So why did he kill Afghans? We learned yesterday that the soldier had recently seen one of his mates with his legs blown off. But so what?

The Afghan narrative has been curiously lobotomised – censored, even – by those who have been trying to explain this appalling massacre in Kandahar. They remembered the Koran burnings – when American troops in Bagram chucked Korans on a bonfire – and the deaths of six Nato soldiers, two of them Americans, which followed. But blow me down if they didn't forget – and this applies to every single report on the latest killings – a remarkable and highly significant statement from the US army's top commander in Afghanistan, General John Allen, exactly 22 days ago. Indeed, it was so unusual a statement that I clipped the report of Allen's words from my morning paper and placed it inside my briefcase for future reference.

Allen told his men that "now is not the time for revenge for the deaths of two US soldiers killed in Thursday's riots". They should, he said, "resist whatever urge they might have to strike back" after an Afghan soldier killed the two Americans. "There will be moments like this when you're searching for the meaning of this loss," Allen continued. "There will be moments like this, when your emotions are governed by anger and a desire to strike back. Now is not the time for revenge, now is the time to look deep inside your souls, remember your mission, remember your discipline, remember who you are."

Now this was an extraordinary plea to come from the US commander in Afghanistan. The top general had to tell his supposedly well-disciplined, elite, professional army not to "take vengeance" on the Afghans they are supposed to be helping/protecting/nurturing/training, etc. He had to tell his soldiers not to commit murder. I know that generals would say this kind of thing in Vietnam. But Afghanistan? Has it come to this? I rather fear it has. Because – however much I dislike generals – I've met quite a number of them and, by and large, they have a pretty good idea of what's going on in the ranks. And I suspect that Allen had already been warned by his junior officers that his soldiers had been enraged by the killings that followed the Koran burnings – and might decide to go on a revenge spree. Hence he tried desperately – in a statement that was as shocking as it was revealing – to pre-empt exactly the massacre which took place last Sunday.

Yet it was totally wiped from the memory box by the "experts" when they had to tell us about these killings. No suggestion that General Allen had said these words was allowed into their stories, not a single reference – because, of course, this would have taken our staff sergeant out of the "deranged" bracket and given him a possible motive for his killings. As usual, the journos had got into bed with the military to create a madman rather than a murderous soldier. Poor chap. Off his head. Didn't know what he was doing. No wonder he was whisked out of Afghanistan at such speed.

We've all had our little massacres. There was My Lai, and our very own little My Lai, at a Malayan village called Batang Kali where the Scots Guards – involved in a conflict against ruthless communist insurgents – murdered 24 unarmed rubber workers in 1948. Of course, one can say that the French in Algeria were worse than the Americans in Afghanistan – one French artillery unit is said to have "disappeared" 2,000 Algerians in six months – but that is like saying that we are better than Saddam Hussein. True, but what a baseline for morality. And that's what it's about. Discipline. Morality. Courage. The courage not to kill in revenge. But when you are losing a war that you are pretending to win – I am, of course, talking about Afghanistan – I guess that's too much to hope. General Allen seems to have been wasting his time.