The Downey case could end up breaking the relationship between Sinn Fein and the DUP. If the fallout is allowed to bring down the power sharing administration then Mr Downey will have done more damage by getting on the plane to London for a private visit than he did in all his years as an IRA activist.
Sinn Fein needs to understand how bad this looks.
They appear to have an inside track to the British government for their activists, and to give nothing in return.
The treatment of John Downey, freed on a technicality as a result of a political deal, undermines any credibility they have in demanding repeated public inquiries into security force misdeeds.
If a bomber who allegedly killed soldiers can have the fear of arrest lifted from his shoulders, then why not soldiers who allegedly killed civilians or republicans?
David Ford, the Justice Minister, is right when he says there is need for an open and transparent process for dealing with the On the Runs (OTRs) and other legacies of the Troubles.
There are already signs that things are moving in the direction of crisis and the slide needs to be halted.
Gregory Campbell has said that if the DUP had known about the deal with Mr Downey and other OTRs, it would not have agreed to share power with Sinn Fein. Jim Allister of the TUV goaded him that since the DUP knew now, they should pull out now.
Nigel Dodds, the DUP deputy leader, then warned that devolution was at risk. That was the signal for Peter Robinson, the first minister, to say that he would resign unless there is a full judicial inquiry into the OTRs issue – who knew what, and when.
Theresa Villiers, the Secretary of State, won't like a gun being held to her head, but she would be wise to agree and halt the drift.
As she admitted, 38 letters have been issued to OTRs since the last General Election in 2010, the same year that justice powers were devolved to Stormont.
Withholding information about who received letters and why will only lead to further tension and speculation. It is a politically volatile moment, in the run-up to European and local government elections, a time when our politicians find it notoriously difficult to bite their tongues.
In return, there needs to be a frank admission that the system of letters telling OTRs they were not liable to be charged was public knowledge when the DUP formed their power-sharing coalition with Sinn Fein.
The Downey judgment records that, following press reports, a statement was issued announcing that the PSNI was reviewing the OTR cases "to determine whether there remains a lawful basis for arrest, having regard to current human rights legislation".
It was also known that Sinn Fein could forward names for consideration and receive letters back saying whether or not an individual was wanted.
In March 2008, just a year after the administration was set up, Jonathan Powell (Tony Blair's chief of staff) outlined the deal in a book. He wrote about providing "letters that Sinn Fein could use to reassure the individuals concerned, that they could return to the UK without fear of arrest".
It was one of these very letters which proved to be Mr Downey's get out of jail free card.
The PSNI had overlooked a warrant in London and the court held that, even though the letter was apparently issued in error by the NIO, it must be honoured.
Sinn Fein would leave the argument there – existing letters should be honoured, and more should be written if needed. "They should stick to that, they should not try and unpick, despite what the electioneering unionists are at now," Gerry Kelly, who ran the scheme for Sinn Fein, maintained.
Things are not that simple.
We now know that letters had been issued up to winter 2012 under devolution, but neither the justice minister nor the first minister were consulted. That makes a mockery of democracy.
It is extraordinary in any administration that an offender should be able to write to the government to ask if there was any evidence against him, and get a comforting answer.
In Mr Downey's case, we know from court papers that he was suspected of the most serious offences. Besides the Hyde Park massacre, these included the murder of two UDR soldiers in a bomb attack in Enniskillen in 1972, which was still being investigated by the Historical Enquiries Team as late as 2008.
Sinn Fein wants such cases to be handled by a quick review of the evidence behind closed doors, but when it comes to offences allegedly committed by the security forces, suspects should be pursued to the ends of the earth and the end of their lives.
Take the Bloody Sunday soldiers. A £200m inquiry found insufficient evidence to prosecute them, but Sinn Fein demands reinvestigation. It is the same with cases like that of Pat Finucane, a despicable murder which has been the subject of numerous inquiries, in which collusion has been admitted and for which one of the gun gang, Ken Barrett, has already been jailed.
It is true that there are unanswered and disturbing questions about such cases, but there are also unanswered questions about many other Troubles killings, the majority carried out by the IRA.
Most aren't solved.
Victims of the Hyde Park bombing or the dead of Bloody Friday are left wondering who knew what and when, just as much as the victims of Bloody Sunday or the Ballymurphy massacre.
It is not easy to say what should happen in every case, but we need a more coherent and level-headed approach. If it's right for Mr Downey and at least 186 other OTRs to get letters saying there aren't warrants, and to walk free then, why are other cases pursued regardless of cost or prospect of success? The Haass process provided one possible solution, with its once and for all re-investigation, followed by a truth recovery process if victims wished it. The example of John Downey, and the secret system that he benefited from, show that the alternative to an agreed approach is unsatisfactory.
Mr Downey has been a supporter of the peace process but, apart from that, society as a whole gained nothing by lifting the fear of arrest from his shoulders.
He told us nothing, no doubt on legal advice, as he walked free.
There was no recovery of information and no holding to account. This should encourage unionists to move forward on the Haass process or agree a better way. The status quo does not serve society's interests and produces regular crises.
Barra McGrory, the director of Public prosecutions, acted as solicitor for the OTRs before taking up his current office with strong cross-party support in 2011.
A spokesperson makes it clear that he has not dealt with OTRs since then but, having seen both sides of the process, he is uniquely placed to make suggestions on how we might deal with the past in a holistic and consistent way.