There are many reasons to welcome the verdict in the trial of Anders Behring Breivik: that he is sane and legally responsible for the murder of 77 people – mostly members of the Norwegian Labour party – on 22 July last year.

The guilty verdict recognises the monstrosity of Breivik's acts, carried out in pursuit of his political beliefs. It also delivers the outcome wanted by the majority of Norwegians, in particular because it means he will spend no fewer than 21 years – and most likely life – in jail. Justice has been done to the fullest extent possible under Norwegian law.

To understand the full import of the outcome, however, one needs to look to the wider realms of politics and society. The trial was dominated by the question of Breivik's sanity for more than just procedural reasons.

Once it was realised a white, middle-class Norwegian man was the culprit and that he'd left a sickening but coherent 1,500-page manifesto for all to read, the race was on for some on the right to depoliticise Breivik's acts. The problem was that his politics were not just similar to their own, but often drawn directly from their statements, cut and pasted into his tract. In many cases the only difference was that he took their language of a war of civilisations to its logical conclusion in violence.

It wasn't just harder rightwingers such as Melanie Phillips, Mark Steyn and Pamela Geller who tried to deny the connection, but many more moderate writers and politicians. This should not be surprising, as Breivik's opposition to Muslims, multiculturalism and a "cultural Marxist" fifth column was never far from the surface in the mainstream discourse of the war on terror. Norway, for all its famed tolerance, continues to be an active part of the Nato occupation of Afghanistan.

The main form this depoliticisation took was the medicalisation of Breivik's actions in terms of psychological or psychiatric pathology. Within days, everyone from forensic psychiatrists to the London mayor, Boris Johnson, felt the need to put Breivik in a diagnostic box. Occasionally, even reportage of his personal history and psychology went to ludicrous extremes to seek his motives in anything but what he actually said. This reached its pinnacle with the first court-ordered psychiatric report, which found him to be suffering from "paranoid schizophrenia" on the basis of clumsy and inappropriate interpretation of ideas and behaviours common in far-right and online gaming subcultures.

Outrage over the findings led the court to take the unusual step of commissioning a second report. This one paid more attention to his political milieu, as well as his behaviour in jail, and found him sane – at most exhibiting signs of a personality disorder. Friday's verdict confirms this conclusion and denies to Islamophobic ideologues the comfort of a clear line of sanity separating their influence from Breivik's actions.

The verdict is doubly important as an intractable economic crisis across Europe provides opportunities for the advance of the far right. Virtually all of these rightwing parties disowned Breivik's actions while singling out in their own propaganda the same groups he targeted. At one extreme, Greece's neo-Nazi Golden Dawn party has seen its electoral support rocket to 7%, even as its thugs carry out violent attacks on migrants in the streets of Athens. But much wider networks of extreme rightwingers populate the internet and organisations of the populist right, exactly the spaces in which Breivik's ideology and commitment to action were formed.

Friday's verdict thus not only delivers justice, it also clarifies the connection between his crimes and how dangerous rightwing ideologies have infiltrated an apparently "sane" mainstream discourse. It is a problem that cannot be expunged simply by labelling it as mad, but must be tackled directly as the political threat to freedom and democracy it is.