What, beyond a sudden insane impulse of rage or desperation or a misguided sense of grievance, would drive one apparently normal person to kill another? It is such an irreversible step into the dark night of the soul that few people would venture into it. That strikes me as a valid enquiry for a novel. Many of the great classic novels of the past, now accepted as general reading, were in effect crime stories. Few of us can write classics, but they provide a standard of excellence. Some writers excel at the genre, but in truth the plots of many modern thrillers are based on straightforward or banal motives, trivializing this grim psychological territory.

Although there have been honourable exceptions, most killers in books develop an acute need for money, or they want revenge for some real or imagined grievance, or they wish to receive a legacy that would otherwise go to a sibling or a disliked cousin, or for various reasons they feel they have to get an unwanted spouse or relative out of the way. They try to cheat insurance companies. They think they can elude the police, or the private detective hired to trap them. They remain, on the surface, civilized and articulate, and in some cases see the evasion of justice as a game.

Several thriller writers have an interest in psychopathy, the motives of a maladjusted serial killer, but as far as my own books were concerned I felt there were only a limited number of ways that could be used as material for a novel.

I was increasingly interested in the ordinary person, one who is invisible in the context of normal society, who ventures for some reason into that dark world of the soul, who then tries to be released from it, or who rejects it, or who dangerously embraces it.

My recent books therefore depended less on plot, murderous gimmicks or a twist ending than they did on character, circumstance, background. I wrote about the conflict created when the protagonist (or the person driven to murder – not always the same) sets up boundaries, or a context within which the crime will take place, but overlooks a crucial fact that will ruin everything.

Sometimes my books ended without the solution the main character was looking for – the crime is unsolved, the murderer gets away with it. A few of the reviewers said they disliked that. I was scolded because crime must not pay. Every puzzle must have a solution, they would sermonize in their reviews, perhaps overlooking the fact that there was a different kind of mystery leading to another sort of outcome, one which I intended to be read as more human, more plausible.