It was a perfect day for such a pilgrimage.

Thin rain was driven by a bitter northerly wind. It had me huddling in an inadequate spring coat as I followed in my grandfather's footsteps through the trenches of Sanctuary Wood outside Ypres, wondering how such a tall man had survived in such shallow, muddy ditches.

Afterwards, though chilled to the bone, I found myself unable to hurry along the tidy rows of white headstones in the Commonwealth War Graves cemetery at Tyne Cot overlooking Passchendaele. Almost 12,000 souls are buried there, 8,369 of them known only unto God. Often their regiment is listed on the headstone, meaning the soldier's uniform was more identifiable than his face.

The many Scots dead lie between men from Lancashire, Northumberland, London and Sussex. The Irish and Welsh lie alongside Australians and New Zealanders. Bounding the cemetery at the top end is a curved wall listing 35,000 who died in the quagmire into which men, horses and guns sank and who have no identified grave.

None of 150 cemeteries in the area glorifies war. Each highlights the pity of it, the waste. The countryside is banal, domestic farmland. It seemed impossible that almost half a million men were killed here in the space of 100 days; and all to gain eight kilometres of this featureless ground.

Inside the Tyne Cot visitor centre a quote from an Aberdeenshire widow is inscribed on the wall: "The thought that Jock died for his country is no comfort to me."

It is a powerful memorial to the madness of war.

I was returning from Belgium on Sunday when I read an interview with the actor Sean Penn saying we don't see enough of the real violence of war, the horror of it. He is one of the millions of Americans who have watched the Islamic State (IS) propaganda videos of hostages being beheaded.

Penn, who was publicising his new film The Gunman, said: "In the 60s we saw the horror of Vietnam on our screens every day. Today we are anaesthetised by political correctness. The American news channels did this with the Iraq war. They wouldn't show what it was about. They wouldn't show the caskets coming home."

Does he have a point? Should we be watching these beheadings? If we don't, are we anaesthetising ourselves?

He is right in remembering the impact of the Vietnam War. In sharp contrast to the First and Second World Wars, families at home could see what their troops were being exposed to. They could also see what they were doing. And they didn't like it. In Europe and America anti-war protest movements put pressure on their governments. Eventually America did quit Vietnam, in defeat.

As a consequence was the American government sparing in its coverage of the death toll from Iraq? Did it aim to better manage public opinion? Probably.

Did it need to be sparing? I'm not so sure.

Vietnam had enormous impact because it was the first televised war. Since then we have all seen plenty of "real violence". We have seen carnage following bombings in Syria and in Palestine. We've seen the remains of the Hutus and Tutsis massacred in Rwanda. We have seen people whose hands were chopped off by rebels in Sierra Leone. We've seen the dead lying rotting in villages razed by Boko Haram. I could continue.

Our minds are so choc-a-bloc with horror that some think we have grown accustomed to it, that our sensibilities are blunted. We are at least adept in blocking out what we don't want to absorb by switching channel or turning the page promptly when it feels like too much.

I certainly thought I knew the worst - until IS. Penn's comment made me wonder why I draw a line between watching some television news footage of war and a beheading in Syria or Iraq. This is my reason why.

I see a marked difference between IS videos and, say, the footage of the naked child in Cambodia filmed running screaming along a road, stripped and burned by napalm. The girl was caught on camera by a journalist whose purpose in being there was to show the world the awful truth. He was a dispassionate recorder of fact. It was for the viewer to draw their own conclusions about war and what it did to children.

The IS videos, by contrast, have been made entirely for propaganda. The beheadings aren't something caught on camera. They are staged. They are choreographed. The victims are in matching costumes - as are the killers. Speeches are made, messages are delivered. And the greatest message is a warning to be afraid.

It equates with the practice of placing heads on pikes and leaving them to rot on city walls.

I would go further. I would parallel the watching of these videos with the watching of child abuse. It too involves filming a helpless victim being traumatised and harmed in a staged setting to be viewed by an audience. Many who watch child porn claim they would never themselves touch a child and that they do no harm because the film and had already been made. They were just watching.

But when the filmmaker is motivated by seeking an audience, isn't watching part of the crime?

And isn't there another danger? It's about letting something evil into your head. I remember some years ago the serial killer Ian Brady issued an invitation to the Scottish crime writer Ian Rankin, who was making a television programme on the nature of evil. He wanted Rankin to visit him. Rankin declined. "Brady was one voice I didn't want inside my head", he said.

It's an important consideration. Watching news footage of carnage around the world, visiting the battlefields of old is about being well-informed, about not taking war for granted. We may feel moved by the experience, as I was at the weekend. We may feel distressed, for example when we see Syria's devastated population. What we don't usually feel is manipulated into being witness to and somehow part of an awful and cruel depravity.

My refusal to watch the IS beheadings isn't just that I don't want to witness that level of violence or to allow myself to be used. It's that, like Rankin, I won't allow that level of evil into my head.

Unlike Penn, I feel we are exposed to more than enough violence. We act it out in video games, watch it in films and read about it in novels and watch it for real on the news. We are exposed to so much of it it's a wonder that we don't forget that there is also beauty and laughter in the world.

That is what I've taken from Passchendaele. If the young men lying in those graves or blasted beyond recognition could speak again, would they say we need more exposure to violence? Or would they urge us to lead the happiest and most peaceful lives we can manage; if only because they couldn't?