We know what former SNP chief Peter Murrell bought with £400,000 of embezzled funds. What I’d like to know is why | Gaby Hinsliff

His guilty plea means motive will for ever be a mystery. It just proves that the world can look as hard as it likes at someone’s marriage and never know what’s going on, says Guardian columnist Gaby Hinsliff

It sounds like the haul of an unhappy trophy wife, filling her empty days with retail therapy. Three Fortnum & Mason advent calendars, seemingly priced for those to whom money is no object; a pair of incomprehensibly expensive Lalique crystal salt and pepper grinders; several hundreds of pounds’ worth of Le Creuset; and no fewer than six Nintendos.

But these aren’t the contents of some influencer’s shopping bags. Rather it’s part of the charge sheet against Peter Murrell, former Scottish National party (SNP) chief executive and estranged husband of former first minister Nicola Sturgeon, who pleaded guilty this week to slowly embezzling more than £400,000 from the party to which they both devoted their lives and blowing much of it on designer luxuries. What we may never know is why.

What made a man who seemed a model supportive husband – happy to stay in his alpha wife’s shadow, running their domestic life and orchestrating election campaigns – take such crazy risks with their joint political project? Sturgeon can offer no answers, beyond pleading that she was as deceived as everyone else and that the subsequent breakdown of their marriage has been traumatic. Essentially, her story is that they had separate bank accounts, and that she was too busy running Scotland to question who paid for their £3,070 robotic lawnmower, or revamped home library. Meanwhile, Murrell’s admission eliminates the need for a trial, which may have unpicked his motives for this steady pilfering that began in the year they married and continued for 12 years. The only clue left is this almost tragicomic shopping list, from which two distinctive patterns emerge.

The first is an either careless or entitled misuse of party money on small things – parking tickets, bottles of the Avon Skin So Soft body spray that Scots swear repels midges – oddly redolent of the 2009 Westminster expenses scandal, as though all boundaries between the couple’s needs and the party’s had become blurred. But the second hints at a more compulsive form of risk-taking.

Who was ever going to admire that blingy peppermill? The Sturgeon-Murrells weren’t great dinner-party givers: outside work, the first minister was a self-confessed introvert, happiest with a book, who says they “rarely” socialised. The fancy coffee machines, fountain pens and cars, meanwhile, are the kind of toys some men buy in middle age either to reward themselves or compensate for something missing. Were the “old money” brands he favoured – the Smythson bags and Bremont watches – a subconscious way of reasserting status, inside a marriage that was unconventional for a man of 61-year-old Murrell’s generation? Or was it, even, stealing for kicks? He must have known it would sink both their careers if he were caught.

But perhaps it simply reflected an unusual partnership in which the roles of spouse and boss were never properly delineated. “I’m the man with the money, I need to buy something,” Murrell reportedly told staff in a Shetland jeweller’s in 2019, during a byelection visit with his wife, before buying her a pendant that would end up catalogued for the courts. What remains unclear is whether that was a romantic gesture, or chiefly about garnering goodwill in a seat where the SNP spent a fortune campaigning.

The political lessons to be drawn here are limited, beyond that people can be surprisingly forgiving of a political movement promising something others won’t. (Though SNP support was down in this month’s Scottish parliament elections, support for independence is, if anything, rising, as Scots contemplate the prospect of being ruled by Nigel Farage from Westminster.)

So, to some of Sturgeon’s more diehard fans, this may still seem the story of a powerful woman repeatedly undone – as she was in the case of her mentor, Alex Salmond – by a man’s sins, and a sexist media holding her responsible for them. But that doesn’t wash here. Any leader would face hard questions about how their party’s chief executive got away with stealing for so long, whether the pair of them were married or not. In the circumstances, it’s hardly sexist to question the wisdom of keeping it all in the family, when the risk was always of precisely what the former SNP MP Joanna Cherry describes: a culture of frosty defensiveness if Murrell’s work was questioned in front of his wife.

What’s left is perhaps only the truism that the workings of a marriage are a mystery, to outsiders and occasionally even to spouses themselves. Unfortunately for Sturgeon, unexplained mystery is the one thing accountable politics can’t abide.

• Gaby Hinsliff is a Guardian columnist