Patricia Cornwell, who writes grisly crime novels but counts herself on the side of the angels — avenging angels, naturally — prides herself on her kindness. She rescues dogs from neglectful pet shops, helps young writers and gives millions of dollars to charity. She even does me a favour by saving me the trouble of having to describe her. Scanning her last novel, Trace, in preparation for talking to her about her latest, the already bestselling Predator, I come across this: “She is an attractive woman in a powerful way, not a big woman but strong-looking, in a midnight-blue pantsuit and midnight-blue blouse that sharpen her handsome features and set off her short blond hair. Her hands are strong and graceful . . .”

This goddess is actually her habitual heroine, the forensic pathologist Dr Kay Scarpetta, but the moment Cornwell enters her suite in the Dorchester I recognise her, even though she is wearing not midnight blue but a grey Armani trouser suit and even though, at 49, she is some years Kay’s senior.

Like Scarpetta, too, she is careful about keeping this well-toned body of hers from harm. I would, for instance, be astounded if the big guy who greets me at the door is, as he is described, her publicist and not, in fact, her bodyguard.

Cornwell, who owns a small arsenal of handguns back in America, has been stalked by fans, some of whom have brought knives and firearms to book signings.

She says that, like many wealthy writers, she attracts “predators”. Does she think she has weird readers? “Definitely. Not only do I have ‘fans’ who get obsessed with me, I have ‘fans’ who get obsessed with my characters, as if they’re real. They get involved in strange activities on the internet.” That is weird, though other, less fanatical, readers of her oeuvres might feel that she is the weirdo.

When in 1990 she published her first Scarpetta novel, Postmortem, Richmond, Virginia, had a high enough murder rate. Since then she has populated it with serial killers with such names as Mr Nobody, Wolfman and, in Predator, Hog, aka Hand of God. Here is one of Predator’s corpses: “The victim has raggedly cut, short black hair that is damp and still gory with bits of brain tissue. There is almost nothing left of her face. It looks as if a small bomb blew up inside her head, which is rather much what happened.”

“Yes,” she says, “I’m graphic about violence. I make it painful. But I do not cross a certain line. In Predator, when you have the hostage situation, I had a difficult time dealing with those scenes because they’re pretty awful, but I could have made them a whole lot worse.”

To illustrate her sensitivity, she tells me how she walked out of a “death investigation school” when a forensic scientist played the class videotapes of a torture. “To witness that pain and horror, the fear, is beyond words. I’ll never, never forget that. I could never do that to my readers.”

The point of her work, she says, is to speak up for the victims of crime. She writes, she suggests, in the tradition of her distant relative Harriet Beecher Stowe, the author of the anti-slavery novel Uncle Tom’s Cabin.

But her empathies lie most obviously with Scarpetta, like Cornwell Miami-born, like her divorced and like her a labourer in the forensic vineyard, although Cornwell’s six years at the medical examiner’s office in Richmond, after a youth as crime reporter on the Charlotte Observer, were spent in its IT department. Scarpetta and Cornwell also have “issues” with their late fathers, though at this point the plot thickens. While Scarpetta says in Body of Evidence that her career in pathology can be traced back to “the terrible crime of my father’s death” (leukaemia, actually), Cornwell traces her own motivations to the psychological abuse she suffered from her father, Sam Daniels, a lawyer who walked out on the family one Christmas Day 44 years ago. “He was very analytical and had a pristine, sharp mind, but his problem was that emotionally he was unable to connect with people, and could be very cruel.” A sociopath? “I don’t know what his diagnosis would be, but he didn’t seem to feel much remorse when he did very harmful things. He wasn’t even nice to me on his deathbed. We knew it was the last time we’d see each other; he grabbed my brother’s hand and mouthed ‘I love you’, but he never touched me. All he did was write on a legal pad ‘How’s work?’ ”

It was, all in all, a lousy childhood. At the age of 5 she appeared before the grand jury in her home town to give evidence against a neighbourhood security guard who “was getting started on some activity that would not have been very good if my brother hadn’t ridden up on his bicycle and scared him away”.

After her father left, her mother moved the family to Montreat, North Carolina, but, unable to cope, was treated in hospital for depression. The foster parents Patricia was sent to turned out to be as cruel as her father; her dog died of neglect. By her late teens she was anorexic. How on earth, from this wreckage, does such a successful adult emerge? She offers two explanations. The first is the Graham family, as in the evangelist Billy and his wife Ruth, who befriended her in Montreat and encouraged her to write. And the second? “The things that happened to me propelled me in a direction of realising that I must be able to take care of myself because nobody else was going to. I didn’t want to feel powerless again. Whether it’s being molested at 5 or being in foster homes, you have no control.”Is it because she fears ceding control that, since the end of her marriage to Charles Cornwell, a college professor 17 year her senior, she has not been prepared to share her life? “No, I am prepared to do it. Actually I’m in a stable long-term relationship that I won’t go into detail about, if nothing else to protect the identity of the other person.”

She is known to have had at least one lesbian affair. I ask if her lover is a woman. “Yes. So to all these people who think that I’m all screwed up about relationships: I’m in one.” For how long? “I feel for ever, that’s as much as I’ll say. But if you’re in a healthy relationship, it’s not about power. You shouldn’t be with somebody who is trying to take away your power. It should be about empowerment.”