It's the fact that the killing of Ronan Kerr served no purpose - even the political purpose of the perpetrators - which made the bombing last Saturday so depressing.
That - not anger, or grief, but a surge of depression - will have been the feeling immediately experienced by many. Other emotions will have then flooded in.
The purpose was presumably to show that 'the struggle continues' - to that extent, it can be classified as a form of attention-seeking behaviour - and to sustain the embattlement of the forces of the state. But the overall effect will be to strengthen the state and its forces.
This is the lesson of the deaths of Patrick Azimkar and Mark Quinsey, which have been widely recalled over the past few days.
They were the two sappers from the Engineer Regiment killed in an attack at Massareene Barracks in Antrim on another Saturday - March 7 two years ago, the day before their scheduled departure for the war in Afghanistan. Constable Stephen Carroll of the PSNI was shot dead in Craigavon two days later.
The names have been cited, but there have been few references to the political controversy raging at the time of their deaths which momentarily seemed to threaten the institutions of state.
It had emerged on March 6 that the Special Reconnaissance Regiment (SRR) had been deployed to the north to provide back-up for the PSNI. Instant uproar followed from nationalist representatives.
The SSR had been formed in April 2005, absorbing the 14th Intelligence Company (the 'Det'). The Det had form in Northern Ireland, having been the lead agency in the collusion with loyalist paramilitaries which led to the deaths of solicitor Pat Finucane and others.
One cross-channel defence correspondent explained two years ago that, 'the skills and experience gained in Northern Ireland by the Det have been passed down to SSR operatives who have since taken a role in the global war against terror.'
The deployment of the SSR would have been controversial in any circumstances. Given that it appeared to contradict the arrangements for control of policing made at St Andrews, it had a distinct potential to destabilise the Agreement.
A visibly angry Martin McGuinness described the arrival of the regiment as "a major threat". The decision was "stupid and dangerous" and had "shaken his confidence" in Chief Constable Hugh Orde.
"The history of the north has shown that many of these forces have been as much a danger to the community as any other group," the deputy First Minister added.
The SDLP said the deployment "raises the issue of who is in control . . . At lunchtime on Thursday, the PSNI were telling the Policing Board the British Army would not be deployed save for bomb squad support. But, by teatime, we learn that British Army recon units are deployed."
DUP policing board member Ian Paisley Jnr hit back. "This is not a matter for the Policing Board," he said. "We're there to hold the police to account on operational reasons. This is a national security issue."
Chief Constable Orde entered the fray, saying that he had asked for extra support to deal with "extremely dangerous people".
He insisted that, "In terms of democratic mechanisms, accountability, we have stuck absolutely rigidly to all of those which were put in place by the St Andrews Agreement."
He promised to discuss the issue with the board at a meeting the following week.
The reason it's difficult to recall the sense of unease - even of alarm - which pervaded pro-Agreement politics at the time is that along came Massareene and Craigavon.
Literally within 24 hours, Mr McGuinness was standing shoulder-to-shoulder with Peter Robinson and Chief Constable Orde, denouncing the killers of the three men as "traitors" and urging anyone with information which might lead to their arrests to take it to the PSNI.
As far as I can recall, the issue of the SRR operating unaccountably in the north has never figured in political argument since.
It has certainly never been cited as a threat to political institutions.
That was the effect then and will be again now. Even from their own point-of-view, the bombers' action will have served no purpose.
The arguments about legitimacy and morality which have been made on all sides since Saturday won't persuade the bombers, any more than such argument has been persuasive to bombers in the past.
The cruelty of the action and the terrible grief it has inflicted won't be factors in their considerations either. But they might possibly consider the practical question: what on earth do they think they are at?