Like other book reviewers, I had to sign an embargo promising not to reveal before today the contents of Hilary Mantel's new short story collection, The Assassination of Margaret Thatcher.

Having done so, I eagerly awaited its arrival. Although the title seemed a little tabloid, it was no great surprise, given how poorly short stories sell. No doubt Mantel and her publisher thought such an arresting name would guarantee attention. If that was their intention, they have certainly been proved right.

Tasteless is the least of the words being hurled at Mantel after The Assassination of Margaret Thatcher, the final story in the book, was published last weekend. Initially intended to run in the Daily Telegraph, it found a home instead in the Guardian when the Telegraph deemed its subject potentially upsetting to readers.

It was a wise call. To judge by the Twitter storm and online abuse Mantel has ignited, her flight of fancy has been judged tantamount to a murderer's charter, with one commentator calling for the police to investigate her.

Mantel, having foolishly compounded her offence by telling an interviewer that she nursed a "boiling detestation" of Mrs Thatcher, is partly to blame for the uproar. A savvier author would have let the story speak for itself. Instead, she has allowed critics to deem her complicit with a fictional IRA killer and an imaginary woman who inadvertently lets him into her flat.

The story is set in the Windsor home Mantel once owned, from whose window she - like her assassin - watched Mrs Thatcher emerge from hospital. Remembering her own sighting of the Prime Minister, Mantel recalled: "Immediately your eye measures the distance." She made a pretend gun with her fingers as she spoke.

As an idea for a story it is dramatic, but the writing is deeply unconvincing, and at times annoyingly opaque. Nor need the squeamish fear: the plot ends seconds before "the widowmaker" is fired. It is a rather ordinary piece of fiction with which to get into such hot water, and yet I would defend Mantel's right to write about anything she likes, no matter how prurient.

If questions of taste were to dictate what fiction covered, good literature would die tomorrow, and with it the vital role of speculation and invention in our culture. And while the title suggests a desire to court controversy, the material itself is tame, given that its target died peacefully long before publication.

Much has been said of how Thatcher's family must feel about such a work, but I suspect they were made far more anxious during her lifetime at the threat real fanatics and terrorists posed to her safety. Now that she can no longer be troubled by anyone's loathing, a vindictive literary attack will be a mere pinprick on skins that must have grown thick as tarmac during her time in office.

Nor should anybody be shocked that Mantel chose to address such a black topic. She is, after all, best known for her depiction of the killing fields of Henry VIII's court, a reign so blood-soaked and paranoid it makes North Korea look as benign as Disneyland. As the rest of the book's contents suggest, Mantel finds modern life threadbare as a source of inspiration. Death, treason and violence are what get her imaginative juices flowing, and her mind feeds best off political treachery, where the stakes are high.

What is fascinating about the furore is not the question of poor taste or Mantel's malignity, but the public sensibility it reveals. The outrage she has provoked is as clear an example as one could find of the way the popular mood has changed in half a millennium. When Henry was slaying his spouses as if he were swatting flies, nobody questioned his authority, or even his ethics. He retained the respect of his people, and none of the monarchs across Europe thought him less of a king because he was also a thug.

Now, though, not only are we appalled at barbaric beheadings in the Middle East, but we are offended by a minor act of disrespect to one who lies in her grave. It may seem an excessive reaction, but it is also oddly cheering. If even those beyond hurt are accorded such sensitivity, we have surely reached a level of civilisation the Tudors (or their modern-day counterparts) would have deemed unthinkable this side of paradise.