Eamon Collins could have been described as the number one thorn in the flesh of the IRA.
Something he did in the past year - which for legal reasons cannot be described here - contributed to a decision by the Provisional IRA chief of staff to offer his resignation to an "army convention" held in Co Cavan last month.
The delegates rejected his offer and re-endorsed his leadership.
The south Armagh man remains chief of staff, although he must have felt diminished by the episode and been very angry at Collins.
A second figure, a leading dissident IRA man from Northern Ireland who lives in the South, might also have had reason to want to kill Collins.
This man left the IRA last year, it is understood, and joined local anti-ceasefire elements to create the group known as the "Real IRA". This group carried out the Omagh bombing on August 15th last year.
He is the man Mr Andrew Hunter, a British Conservative MP, was referring to last week when he said he was thinking of revealing, under Westminster rules of immunity, the name of one of the people responsible for the Omagh bombing.
This man was one of Collins's close associates in the Provisional IRA in the 1980s and was suspected of involvement in a number of murders of security force members in Northern Ireland during the late 1970s and early 1980s.
He was subsequently a leading member of a special unit the Provisional IRA kept in the south Armagh/north Louth area to abduct, interrogate and, if necessary, torture and murder IRA members suspected of being informers. This group carried out most of the tortures and assassinations of suspected informers during the 1970s and 1980s.
A small number of other close associates of Collins who are also believed to have wanted to kill him. These include at least three other members of the Newry Provisional IRA unit who worked closely with him in a number of murders in the town in the early 1980s. They had left the town when Collins became a State witness, but would now be free to return.
Another possible suspect is a man from south Armagh who was the head of the IRA in the area and who would have felt threatened because he had known Collins so well.
Gardai suspect that Collins's murderer was one of his former Provisional IRA associates. None of the IRA in Newry, south Armagh or Dundalk had apparently forgiven Collins for his decision to turn State's evidence and then commit his story to print.
In April 1979 he was run over by a car, in another attempt to kill him, but he escaped with some broken bones and bad bruising and had difficulty walking for some time.
Collins lived in Newry, not far from where he was found dead yesterday morning. There had been regular threats to his life and incidents of intimidation, including the spray-painting of "traitor", "scumbag" and other graffiti on his home last week.
Last September his enemies burned down his family's farmhouse in Camlough in south Armagh. He had been restoring it and had intended to move there when it was completed.