THE best horror films – the ones that keep you on the edge of your seat – always involve choices. The choice between investigating the noise upstairs or fleeing to a neighbour’s house. Between screaming for help or hiding in a cupboard. Between what the viewer sees as a sensible decision, and the brave but potentially suicidal alternative.

Watching Nicola Sturgeon starting an “open discussion on the direction of income tax in Scotland” yesterday, I thought I glimpsed a sinister shape in the background. Was that the Brexit bogeyman, lurking in the shadows, waiting to disembowel the Scottish economy? Or was it merely Ruth Davidson, still in guising garb, ready to pounce as soon as the Unionist trap was sprung?

It would be unfair to compare the UK Government to the serial killer in the Saw movies – whose ideologically driven killing sprees take the form of elaborate games involving knives, syringes, iron maidens and rusty tools – but handing income-tax powers to the Scottish Parliament was a stroke of evil genius. It presented the Scottish Government with an instant, self-contained dilemma: use the new powers to raise taxes and risk losing votes, or decline to use it and risk accusations of hypocrisy any time Tory austerity and public-service cuts get a mention. You wanted more powers? You got more powers. Heads you lose, tails you lose.

This isn’t quite as stark a choice as, say, whether or not to hack off your own foot, but in political terms it could still prove seriously damaging. “Pay more tax for the same standard of living” is not a winning campaign slogan, even if it’s the practical reality with which we’re faced thanks to the reckless decision of the UK electorate to stand in front of a mirror and say Boris Johnson’s name three times. Much will now depend on how Sturgeon and her colleagues dress up the inevitable decision to raise income tax, and how they justify reneging on a manifesto pledge made a mere 18 months ago.

Of course, an awful lot has happened since then. The SNP lost their majority, the UK voted to leave the EU, the Tories lost their majority at Westminster and now the grubby pasts of UK Cabinet ministers are coming back to haunt them, creating yet more political chaos. Theresa May can’t afford to lose any of her precious MPs, so she might be forced to drag along the odd rotting corpse when she takes another stomach-churning ride on the ghost train of Brexit negotiations.

Brexit alone would arguably be justification enough for a change of tax policy, but Sturgeon is very keen to emphasise that this is not her party’s free choice to make.

If Patrick Harvie wants to climb into a dusty attic and root around for coins to fund a huge public-sector pay rise, she’ll have little choice but to follow him up the ladder with a torch. If the SNP, Greens and Labour all manage to agree on a tax-raising mechanism, then all three parties will have to share the blame if the electorate get spooked.

So will the Scottish Government’s discussion paper spark a good-faith debate about the kind of country we want to live in, and the price we’re willing to pay for it? The First Minister and her Finance Secretary Derek Mackay are clearly hoping those who’ve benefited from free childcare, free tuition fees and free personal care will take a look at the sums their families have saved and agree that a modest hike is justified. But what about those who don’t personally receive any of these benefits, and possibly never will? And what if many of those who have personally done very well out of 10 years of SNP government nonetheless object to being asked, at this stage in the game, to chip in a bit more from salaries that have stagnated over the same period?

A purely individualistic approach might seem the safest bet for a party with the SNP’s broad support base – including as it does everyone from radical socialists to indy-backing right-wingers – but it’s in sharp contrast to much of the language of the independence campaign.

Even if the correct question to ask is “what’s in it for me?”, it can’t be answered simply by totting up the savings from the universal benefits packaged as “Scotland’s social contract” and disregarding the value of a well-resourced NHS, high-quality education for all and a justice system worthy of the name.

Scotland’s universal benefits weren’t introduced simply to save middle-income taxpayers a few quid. For those on the lowest incomes, free prescriptions can mean the difference between taking medicine and going without, and a free bus pass the difference between visiting a friend and becoming trapped in a cycle of social isolation.

It’s impossible to put a price on the long-term benefits of such public services as intervention by social work to protect children from abuse, or halt a spiral of offending, or prevent an elderly person from being neglected, but these behind-the-scenes services, targeted to those in greatest need, must be protected from the worst effect of shambolic Tory rule of the UK. Raising income tax certainly isn’t the fairest way to do it – and only time will tell what behavioural changes will actually result – but the scary reality is that right now, it’s the only option we’ve got.