Jerry McCabe and his Garda colleague Ben O'Sullivan were sitting in an unmarked Ford Mondeo in the village of Adare, Co Limerick, on June 7th, 1996 when their car was suddenly rammed from behind by a Pajero jeep. Three men in dark balaclavas and black-and-green army camouflage jumped out of the Pajero, and started to fire into the Garda car.

The men seemed utterly determined to kill both of the detectives. One ran to the driver's side and poured bullets from his Kalashnikov automatic rifle at Ben O'Sullivan. Another ran to the passenger side and shot Jerry McCabe three times, killing him almost instantly. In all, the gunmen fired at least 14 shots. They gave no warning and showed no mercy. They didn't even pause to grab the money from the post office van that the two detectives were guarding, but sped away at once. If it was possible to make these actions worse, what happened in the run-up to the trial of the killers in January 1999 added insult to the rule of law to the terrible injury to the McCabe and O'Sullivan families. The IRA effectively managed to force the prosecution to accept a plea of manslaughter rather than murder from the accused by striking fear into the hearts of key witnesses.

One man who testified that one of the accused was similar to a man he had seen in Adare on the day of the shooting said he had been "intimidated and threatened". Another key witness accepted an 18-month prison sentence for contempt of court rather than give his evidence. According to his solicitor, he was put under pressure which led him to fear for his life. In the light of all of this, it is very hard to stomach the notion that the men who killed Jerry McCabe and tried to kill Ben O'Sullivan should now walk free under the terms of the Good Friday agreement, as Sinn Fein has been demanding. For the families involved, for the Garda Siochana, for the State and for the vast majority of the Irish public, such a prospect is quite sickening. It would seem to make a mockery of the most basic principles of morality, justice and law-bound democracy.

And yet, Jerry McCabe's killers should be released now, not just for pragmatic political reasons but for good moral reasons as well. Justice isn't only about retribution. It is also about consistency. To treat those who are guilty of similar crimes in fundamentally different ways is unjust. To demand that others bear burdens we ourselves regard as unbearable is immoral. If some of the people who were released from the Maze prison in Northern Ireland last week are to escape the full retribution for their actions, then so must the killers of Jerry McCabe. If the Republic is to demand - as it does - that people in Northern Ireland must stomach the release of vicious killers for the sake of the Belfast Agreement, it must be willing to swallow the same bitter pill.

The awful reality is that there was nothing unique about the cold-blooded cruelty of Jerry McCabe's killers. The IRA, no more than the UDA/UFF, the UVF or the INLA, was not in the business of compassion and mercy. The work of the terror gangs was murder and maiming and everyone who supported the Belfast Agreement knew that part of its price was the release of people who, almost by definition, had behaved savagely. One of the UFF men who walked into the Rising Sun bar and restaurant in Greysteel in October 1993 wearing similar gear and carrying similar guns to the balaclavas, fatigues and Kalashnikovs of Jerry McCabe's killers and murdered seven innocent people openly laughed at the relatives of the dead on his way in to his trial.

But those men have been released under the agreement. The man who helped to beat 16-year-old James Morgan to death with a hammer in July 1997 and dump his partially burned body in a pit used to dispose of dead farm animals has been released. So has the man convicted of assisting with the murder of postal worker Frank Kerr in Newry in November 1994, when the IRA was supposedly on ceasefire. So have the men who beat and kicked to death Constable Gregory Taylor as he emerged from a bar in Ballymoney in June 1997. So have dozens of other men guilty of unspeakable crimes, some of whom have shown not the slightest sign of remorse. Yet the families of many of the victims of these awful crimes voted for the Belfast Agreement in the full knowledge of what it meant for the killers of their loved ones. James Morgan's father expressed the feelings of many of them when he said "It wasn't easy. I had mixed feelings about it, but we voted yes. Whether his murderers are in and out of prison, it won't bring James back. At least if it stopped this sort of thing, it would be worth it."

If people who have suffered such unimaginable pain can come to a rational decision to sacrifice the right of retribution to the demands of peace, then so can the Republic as a whole. Aside from the pain that it would cause Jerry McCabe's family, the strongest argument against the release of his killers is the message it would send to Irish society as a whole. But what message does a refusal to release them send in the context of the dozens of other early releases? It inevitably suggests that there are different categories of victims in the bleak history of the Troubles. If some killers are singled out for special treatment, the inescapable implication is that some deaths somehow matter more than others. Contrary to their own best intentions, those who make this kind of argument actually fall in with the logic of the killers themselves.

The organisation responsible for the killing of Jerry McCabe was besotted with the notion of "legitimate targets", sifting human beings through its own warped categories into proper and improper objects of its annihilating contempt. To suggest that what happened in Adare in June 1996 was somehow of a higher order of crime to what happened in Greysteel three years earlier or in Ballymoney a year later is to engage, however inadvertently, in the same kind of reasoning.

For the one thing that makes the killing of Jerry McCabe exceptional is that he was not, in the IRA's own terms, a legitimate target. By its own rules, the IRA was not supposed to murder members of the Republic's security forces. But to accept that this really makes some kind of fundamental difference is to take one's morality from the IRA. It is surely far better, and far less dangerous to democracy and the rule of law, to take it from the Belfast Agreement and its open, honest, clear-eyed decision to choose the lesser evil of early releases over the much greater evil of endless violence.