Garvan Walshe is a former National and International Security Policy Adviser to the Conservative Party. He now runs Brexit Analytics.

The death earlier this week of Martin McGuinness, IRA terrorist turned deputy First Minister of Her Majesty’s devolved administration at Stormont, closed the cycle opened in 1886 when Joseph Chamberlain took the Liberal Unionists out Gladstone’s party, furious at the concessions it made to Irish nationalism in political form.

McGuinness died unpunished, and without expressing remorse for the killings, maimings and the disappearances the IRA carried out. If we remember that his organisation never stooped to suicide bombing, we forget that it would kidnap men’s families, and force the men to drive trucks full of explosives into an army base. At least Al Qaeda had the decency to recruit volunteers for its “martyrdom operations.”

McGuinness turned political defeat into personal victory, presiding, however improbably, with Ian Paisley over a power-sharing administration in Northern Ireland that froze out the moderate Ulster Unionists and SDLP. That this might have been necessary to bring peace to the province doesn’t prevent us from observing that he got away with it.

As I write, other terrorists have attacked the same Parliament in which Sinn Fein refused to take their seats, and in which the INLA murdered the MP Airey Neave in 1979. We don’t know who they are. The speculation, inevitably, is that they are, in some loose or less loose way, affiliated to ISIL.

The police responded efficiently and and bravely. At least one officer lost his life so that the the politicians who compose our representative institution and the thousands of staff who support their work can serve the public.

In the days to come we will naturally turn to examine the terrorists’ motivations and ideology; their psychological profiles and history of radicalisation. This is to be understood and indeed welcomed. Terrorism has a long history, and if we think terrorists come out of nowhere that’s only because we haven’t been looking in the right places. That terrorists fuse the criminal’s self-justification and the radical’s romantic call to action is hardly psychologically exceptional.

But every terrorist – lone individuals like Thomas Mair as much as self-styled military commanders like Martin McGuinness – enters a plea of diminished responsibility. They pretend to be “driven” to commit their murders by the action of others. If their victims are, by normal standards, deemed to be noncombatants: civilian police officers, children unfortunate enough to be near a bomb as it explodes, a Member of Parliament in her constituency; the terrorist first shifts the blame. I, they assert, am just the instrument. Whereas you, the state, are at fault for creating an injustice so serious that only by killing the innocent can I bring it to your attention. You, the terrorist insists, are the only one with agency; I, the terrorist, have none.

The terrorists find enthusiastic support from the Blowbackists, who insist an attack is never the responsibility of the attacker, but blowback from the system that created the conditions that caused (note the passive voice) the terrorist to act. Adherents of this idea are like expert witnesses giving evidence in support of the terrorist’s diminished responsibility. Often because they have some sympathy for the terrorist’s aims, they deliberately conflate, or at least are negligent in confusing, the terrorist’s own self-exculpation with the truth. Blowbackism appeared even in the week after 9/11, and will in due course find its voice in response to yesterday’s assault.

McGuinness’s own life shows the power that plea of diminished responsibility can wield. Of course, the authorities against which terrorists fight are in a position to create conditions propitious for the growth of terrorism; to the extent that those conditions are wrong, they deserve the blame for them. What they don’t deserve is to be held responsible for the atrocities that men and women possessed of free will decide to commit against ordinary members of the public.

McGuinness died escaping his share of responsibility for the IRA’s campaign of murder. If indeed he was as important in persuading the Provisionals to abandon violence as Tony Blair says, that effective political leadership stands to his credit. But it never came with an acknowledgement of the crimes that put him in that position. McGuinness may have been a criminal who sent straight, but he never served his time.