Terrorists use violence to achieve their political goals - those goals are supposed to separate them from common criminals. Religious terrorists are supposed to employ violence in the name of some higher cause, perhaps even according to divine will. It can be argued, though, that the line between terrorism and common thuggery is not so distinct.

The Economist notes:

After all, three things have long been known about Sinn Fein. First, it is joined hip-and-thigh with the IRA, a terrorist organisation that has killed more than 1,700 people since 1970. (The Irish government has outed Mr Adams as a member of the IRA's seven-man army council, along with another leading light of Sinn Fein, Martin McGuinness.) Second, Sinn Fein-IRA has an anti-American bent. Sinn Fein led a campaign against giving American military aircraft landing rights at Shannon international airport during the Iraq war, for example.

Third, the IRA has connections with other terrorist organisations. A senior congressional aide says the “bloom came off the IRA's rose” in 2001 with the discovery that the organisation was training terrorists in Colombia. “This has nothing to do with the British empire or national liberation,” he says. “This is our own backyard.” Colombia supplies most of America's cocaine and heroin, and its terrorists have been holding three American citizens hostage for several years.

It is one thing to forgive the IRA for “collateral damage” in a national liberation struggle. But what happens when its people slit open the abdomen of a fellow Catholic from navel to breast bone, sever his jugular vein and then gouge out an eye—as happened to Mr McCartney? It is one thing to dismiss everything the British government says as imperialist propaganda. But what happens when the Irish prime minister in Dublin accuses the IRA of criminal activities?

The IRA is an organization created to achieve certain political goals through violence. It is also one that has largely defined its struggle as much in religious as in nationalistic terms — indeed, the two aren't even always considered separate. Once you successfully use violence to achieve your political or religious goals, though, what reason is there not to use violence for other goals?

The above events seem to indicate that at least some in the IRA don't see any reason not to continue with violence as a general tool. Want independence? Use violence. Need more financing? Use violence. Bothered by someone in a bar? Use violence. Once you start it's difficult to justify stopping and this is one reason why the demands of terrorists shouldn't be acceded to.

Read More:

• War on Terrorism
• Islam, Violence, and Terrorism