Sean O'Callaghan, one of the most important defectors from the Provisional IRA, successfully evaded the republican movement in death as in life by dying on Thursday of natural causes.
Much has been written in the obituaries about the amazing details of O'Callaghan's journey - from the precocious, murderous, republican "boy soldier" of the 1970s to the double agent extraordinaire who saved the life of the Prince and Princess of Wales from an IRA bomb, which was due to be planted in a lavatory next to the Royal Box at the Dominion Theatre in 1983.
What, though, was the wider significance of his career?
The title of his autobiography is The Informer. And he was an informer in every sense of the word: first, for his handlers in the Garda and in MI5; but, subsequently, also in the wider sense, by informing key decision makers in the United Kingdom (such as Tony Blair's chief of staff Jonathan Powell, and David Trimble) about the nature of the republican movement after his release from prison in 1996.
Indeed, his wide circle of friends in London after his release ranged from the Marquess of Salisbury through to Tom Baldwin, the former Times journalist and senior aide to Ed Miliband.
O'Callaghan's advice was particularly important to Trimble, giving the Ulster Unionist leader extra confidence to join the first power-sharing Executive involving Sinn Fein in 1999, though he felt that the party had lost grasp of key details of the Belfast Agreement, such as the Patten Commission on the reform of the RUC.
O'Callaghan had the highest opinion of the RUC's professional abilities, and believed it deserved better than Patten gave.
O'Callaghan was certainly not the only senior republican to work for the security forces - the subsequent exposure of figures such as Freddie Scappaticci illustrates just how heavily penetrated the Provisionals were - but he was the most high-profile defector publicly to repudiate the IRA on ideological grounds.
A key moment in his intellectual development came in the mid-1970s when he read Conor Cruise O'Brien's revisionist text States Of Ireland. Physical force republicanism in the Troubles, far from being an aberration from the culture of the Irish Republic, was the inevitable outgrowth of the crude Anglophobic grievance narratives about "800 years of British oppression" that formed part of the normative teaching of history in mainstream schools in the south.
As a former high-level Sinn Fein activist before and after the hunger strikes of 1981, O'Callaghan was acutely aware that ideological struggle was as important as armed struggle.
He sought to educate a new generation in Westminster and Whitehall to be as relentless about political warfare as republicans were. As he saw it, republicans were not 10-foot tall (as some unionists were inclined to believe in their more self-pitying moments), but simply never gave up on their objectives.
O'Callaghan continued to push his beliefs with huge courage after his release from prison.
Two examples stand out: first, going back to the Irish Republic, where he was a marked man, to be a key witness for the Sunday Times in its successful defence of a libel suit brought by the senior IRA activist Thomas 'Slab' Murphy.
Second, he rebutted the idea that Pat Finucane - a Belfast solicitor murdered by loyalists in 1989, thus occasioning accusations of State collusion with the UDA - was just a regular human rights lawyer.
In the pages of the Daily Telegraph, O'Callaghan described how he had attended a high-level IRA meeting involving Finucane.
Indeed, O'Callaghan reported that he was the last prisoner visited by Finucane before he was slain; Finucane was far more interested in what information O'Callaghan had given to the police than in his client's legal defence.
Despite O'Callaghan's services to the Irish State, there was much ambiguity - even in respectably bourgeois circles in the south - about his activities as a "tout".
Some of them sought to throw doubt on the value of his testimony. These allegations were refuted by the late Taoiseach Garret FitzGerald, writing in the Irish Times in 1997, who attested to the value of his work.
Lord Bew of Donegore - one the leading historians of Ireland, who later became friendly with O' Callaghan - has suggested that this high-grade intelligence inadvertently played its part in the Anglo-Irish Agreement of 1985, the greatest defeat suffered by unionism during the Troubles. Some Irish ministers and officials implied to their British counterparts that many more nuggets would come London's way if only Mrs Thatcher would give the Republic a formal role in the government of Northern Ireland. Thatcher signed up, but was disappointed in this aspect of the 1985 accord. There was only one O'Callaghan.
O'Callaghan remained truculently independent to the end, refusing official offers of protection, believing he could look after himself better than the authorities could.
He disliked aspects of the "securocratic" approach - particularly the "moral equilateralism" of some panjandrums of British intelligence. He recalled an MI5 officer (who debriefed him in Holland at the request of the Garda Siochana) saying: "If Gerry (Adams) could sort out some of his problems with his people, and we could sort out our problems with Margaret (Thatcher), we might get a deal."
He also believed that MI5 had not stood by the RUC in its hour of need. Despite their hardline reputations, securocrats were sometimes willing to pay an unacceptably high moral and political price to bring terrorist campaigns to an end.
More recently, O'Callaghan pondered the lesson of all this for Islamist terrorism, as he spent much time with Somali and other angry youths in danger of radicalisation.
He contended that the willingness of the British State to indulge historical inquiries in Northern Ireland and beyond would make it harder for future generations of informers (and future generations of agent handlers) to come forward and take the necessary risks to defeat armed struggle.
He knew better than anyone how difficult an informer's existence could be. But his value to two democratic states was huge.