About twenty to four on March 20th, 1989, Finbarr King heard the ugliest sound in the world: the howl of triumph that goes up from a human wolfpack just after it has hunted down and killed its prey.

It is a noise that sounds beneath the beautiful music of civilisation: the great primal exultation over the bloody corpse of a dead Jew, a dead Black, a dead Serb, a dead Muslim, a dead Tutsi, a dead Papist.

Finbarr King was unlucky enough to hear it on a March afternoon near the Dundalk-Newry road, just across the Border, close to to McGeough’s garage, where he worked as a mechanic.

He was coming back with his colleague Packie O’Hanlon after fixing a broken-down truck near the old customs post at Carrickagh when they were stopped by an armed man in combat gear and made to get out and lie face-down on the grass.

Minutes later, the red Vauxhall carrying the police officers Harry Breen and Bob Buchanan approached the same roadblock and was ambushed by the IRA. Finbarr King heard the shooting in which the two men were killed. But he also heard something else. After Harry Breen, unarmed and waving his white handkerchief, was shot in the back of the head, the IRA men got back in the van in which they had arrived. And, as King told the Smithwick tribunal, "as they were exiting the area, they let out a big roar like 'hurray', or whatever".

Documentary traces
That big roar, the explosion of pure joy at the triumphant conclusion to the hunt, is not often recorded in the documentary traces of the Troubles. Michael Harding heard it in Fermanagh in February 1985 when the IRA bagged Jimmy Graham, a Protestant small farmer and part-time member of the UDR, as he was driving the primary school bus to the local swimming pool. They had previously hunted down his two brothers and now they had finally wiped out the Grahams. Harding recalled, in Colm Tóibín's book Bad Blood, hearing the roars from their van as the IRA men sped back across the Border to safety: "Ya-hoo! Ya-hoo! Ya-hoo!" No doubt many a loyalist gang who had caught and murdered a Teague gave vent to the same inchoate bawl. It is a universal noise, beyond all differences of culture or language.

The big roar that King heard after the murders of Breen and Buchanan is inarticulate but oddly eloquent. We need to attend to it because it gives the lie to the story that Sinn Féin would have us swallow about those killings and so many hundreds more.

The story is that the killers were acting out of a painful sense of duty. As Pádraig Mac Lochlainn put it to Vincent Browne last week, "they were all caught up in a profound tragedy". They took on the burden of duty: "Every soldier and combatant in a conflict has a duty to prosecute a war, tragically, yes, that is the case."

Even in its own terms this is, when applied to the murders of Breen and Buchanan, a lie. Soldiers in conflicts do not have a duty to kill unarmed opponents who are clearly surrendering. They have, on the contrary, a legal duty not to kill such opponents.

Just last week, a Royal Marine sergeant, Alexander Blackman, was given a life sentence by a British court martial for killing a wounded Taliban combatant in Afghanistan.

Or, to look at it from a different angle, Sinn Féin's logic of a duty to kill unarmed enemies would entirely justify, for example, the shooting dead of three unarmed IRA members in Gibraltar in March 1988 as they tried to surrender. I must have missed the statement from Gerry Adams that the SAS members who committed those murders were doing their duty.

But all of this is to appeal to rationality and the roar of joy that King heard that day comes from somewhere far beneath the rational. It rises up from the darkest parts of the human soul. There is a place beyond civility and morality, beyond compassion and sympathy. There are circumstances throughout history in which many otherwise ordinary people come to inhabit that place. They find within themselves a capacity, not just to do terrible things to other human beings, but to be thrilled and exhilarated by those acts. The men who killed Breen and Buchanan knew that terrible euphoria.

Moral debt
These things cannot be undone but they must not be lied about. One of the basic tasks of civilisation is to stop ourselves from falling into this abyss by knowing where its edges are. Sinn Féin and the IRA owe Irish society an immense moral debt that can be repaid in some small part by not perpetuating the cant that blurs those edges.

One of the absurdities of the Troubles was the BBC’s use of actors to overdub interviews with Adams and his colleagues, so that we saw the lips moving but heard another voice.

Until Sinn Féin finds a capacity to speak honestly, many of us can’t help hearing, every time it speaks, a ghostly overdub: Ya-hoo! Ya-hoo! Ya-hoo!