“LET no-one make the mistake of thinking that this was a crime of passion," said judge Justice Turner earlier this month, in sentencing Sarah Williams and Katrina Walsh for the murder of Sadie Hartley. "It was a crime of obsession, of arrogance, of barbarity, but above all else, this was a crime of pure evil.”

Williams had been having a sexual affair with Sadie Hartley’s partner, Ian Johnston, on and off since 2012. Johnston had viewed the affair as “entertainment and purely sexual”. In contrast, he described his relationship with Sadie Hartley as “cocoa and Kindles”, his rationale for this being that “they were nearly 60 and wanted to settle down”. Cake and eating it might be a more succinct way to define his approach to intimacy, splitting aspects of his own need and desire and dividing them, if not equally, distinctly, between two women.

Not a wholesome recipe for a good relationship, but not one that usually ends in murder, either. The trouble is that for personality-disordered individuals like Sarah Williams, who, by all accounts seems to fit the profile for a narcissistic psychopath, sharing doesn't come easy.

When interviewed after the trial ended on August 17, Johnston said: “I didn't for one minute have any inkling that any of this was being planned and even if I had, I've been reassured by police, there was nothing I could do to stop this.”

This is probably true. In order to understand how lethally disturbed Williams was, Johnston would have needed to stand well back and, with a cool head and clinical eye, observed her behaviour, how she regulated her moods, whether or not she displayed empathy towards others or showed remorse. Such measurements are difficult in the heat of a sexually charged, illicit affair. Unfortunately for him and tragically for Sadie Hartley, he couldn't see the wood for the trees. Instead, having got up close and personal with a psychopath, he was unable to see that embedded in Williams’s obsessive love was the potential for murderous rage. Like many people who are infatuated, he probably only saw what he wanted or needed to see. The calculating and grotesque brutality displayed by Williams and Walsh in executing their plan push the boundaries of our wildest imaginings.

The experience of love or lust at first sight is a powerful one. Research suggests that the stronger this feeling is, the more extreme the reaction when a person is rejected by the object of their love. In part, instant attraction happens because the obsessive personality sees their love object as similar or the same as them. In this respect, they actually fall in love/lust with themselves and the "love" is essentially narcissistic. While extreme narcissists appear self-assured and worthy of love, deep down they are insecure and self-loathing. Unable to bear such feelings and accept that, like most people, they are flawed and imperfect, they demand constant and unflinching adoration from others in order to counteract their lack of self-worth. Their egos are brittle and easily shattered. Extreme narcissists don’t "do" rejection. The iconic scenario is probably the "bunny-boiling" episode in the film, Fatal Attraction. When the Michael Douglas character, Dan Gallagher, rejects his lover, Alex Forrest (played by Glenn Close), she rapidly descends into emotional and psychological chaos. Her self-image of perfection and irresistibility feels so under attack that she is prepared to kill in an effort to keep it intact. Her perverse bunny-boiling is, in the character’s mind, a rational and justifiable act. As such, she feels no remorse and cannot accept responsibility for hurting others.

Similarly, Sarah Williams denied responsibility for the murder of Sadie Hartley. Williams showed no remorse throughout the trial. It is likely that, in her mind, her actions were justified given the deep wound to her ego. From Williams’s responses to cross examination, it is clear that she is intelligent, resourceful and manipulative. Without doubt, she is a pathological liar who regards herself as victim rather than perpetrator.

For the psychopath, the distress of others is a foreign and insignificant land. It is possible that with a 30-year jail term ahead of her, Williams may be inclined to explore the dark interior of her mind. If she does, it is likely that what she encounters there will be terrifying and disturbing. Then, and only then, will she have the potential for remorse.