Once in a country childhood I saw a farmer slaughter a sheep. I wasn't supposed to see it. To this day the farmer doesn't know I did.
Had he known a kid from across the road was right behind him at the time, he would have postponed the chore. But I'd happened to wander around the side of his woolshed and glanced in just as he was straddling the creature.
A moment later, as I was still wondering what on earth he was doing, he flung an object behind him. When it stopped rolling I was looking at the calm face of a severed head. Quickly I backed out of there. It's something you never forget.
No one needs an experience like that to have felt the full horror of Wednesday's television news. The picture preceding the decapitation of an American captive in Iraq ought to have been enough.
The fuzzy, jerky images from a jihadi website showed a young man, a civilian construction worker it turned out, sitting on the ground with his back to a line of hooded captors. Directly behind him one of the hoods read a statement before slowly drawing a knife from his tunic.
What followed, in the judgment of almost all Western television channels, was too sickening to put on screen and the bare description so unspeakable that it brought little further comment that night and over the following days.
It is hard enough for non-hunters to imagine what it takes to kill an animal in cold blood, let alone a human being.
How does someone suspend all fellow feeling to slice the throat of a helpless person?
Religious mania perhaps. "God is greatest," these maniacs cried as they put the knife through and held up their bloody trophy.
But their tactics are not mad; they know exactly what they are doing. The statement presented the execution as revenge for the sexual humiliation of Iraqi detainees by American soldiers at Abu Ghraib prison in Baghdad, although they know, I suspect, that the crimes bear no comparison.
Unco-operative prisoners in Abu Ghraib were sexually humiliated and violated, to the evident glee of military guards photographed in some strange activities.
The victims were at least hooded during their humiliation; their tormentors were not. The murderers of Nick Berg were hooded; he was not. We were intended to see the American scream, plead for his life, die in terror.
Comparisons of cruelty are always invidious. Sexual assault is undoubtedly more devastating than might be imagined. The physical invasion must be detestable enough, but the injury to personal dignity, confidence and self-respect is said to be much deeper.
Sexual crimes, more than any other, force the victims to co-operate in their degradation and can even leave them with a sense of shared guilt for their own mistreatment. The shame is particularly acute in Islamic society.
But the victims of Abu Ghraib are alive. Nick Berg is not. His headless body was found on a Baghdad highway overpass.
The atrocities bear no more moral equivalence than the events of September 11, 2001, bore with United States' conduct in the Middle East at that time. Which is to say, the acts are not comparable but it would be idle to deny they are connected, as American Senator John McCain did on CNN on Wednesday night.
The next day McCain's Senate committee saw an even worse collection of photographs and declined to release them for fear of inciting further reprisals - so perhaps they know.
It is probably telling that after recoiling from the latest horror the West went back to discussing Abu Ghraib. The sexual crimes committed there may become the lasting metaphor for the Iraq invasion.
They are not, interestingly, the worst that has happened to the coalition's detainees. The US military has had cause to investigate 25 deaths in its custody over the past 16 months. But it is the sexual assaults that have seized world attention, deeply embarrassed the US, antagonised Arabs and further energised the jihad.
With the beheading of Nick Berg, the self-claimed handwork of Abu Masad al-Zarqawi, an ally of Osama bin Laden, attention just might turn back to the genuine threat to Western security. Iraq has been a disastrous distraction, not only because the invasion deflected attention and effort, but because it was a surrender of the moral high ground.
The US attacked Iraq without provocation, and the motive was always transparent. The shock of September 11 sent the US on the hunt for all possible enemies.
Washington insiders have since confirmed that one old enemy, Saddam Hussein, had been the Bush Administration's obsession from the beginning.
After September 11, Iraq presented a doubly appealing target. Saddam may have had nothing to do with al Qaeda but no matter. To remove him the US could march right into an Arab state. That would show them.
It was madness but it's done. The US begins the attempt to pull back next month, for better or worse. The killing of foreign reconstruction workers, including a New Zealander this week, are just parting shots.
One of those killings was filmed for our sake. The killers act in the name of seventh-century beliefs but they can use jet aircraft and, now, cyberspace for their own purpose.
The staged murder of Nick Berg was a reminder the cancer within the Islamic world is still spreading unchecked. It is as abhorrent to the majority of Muslims as it is unintelligible to everyone else.
This was just to say again that they know our sensibilities, they know how to hurt us.