Brian Blackwell, the 19-year-old who killed his parents and then went on a holiday financed by their credit cards, was diagnosed as suffering from narcissistic personality disorder.
Psychiatrists were unanimous in their assessment of Blackwell
The disorder is named after the mythical character Narcissus who fell in love with his own reflection.
It results in the sufferer being obsessed with the fantasy of unlimited success, power, brilliance and ideal love and beauty.
Mental health groups say it is rare that the disorder leads to murder.
But sufferers can be confrontational, believe their problems are unique, and fly into a rage if their inflated sense of self-worth is challenged.
Grandiose sense of self-importance
Preoccupied with fantasies about unlimited successes
Believes self to be "special"
Requires excessive admiration
Has a sense of entitlement
Is interpersonally exploitative
Is often envious
Displays arrogant, haughty behaviour
Dr David Holmes, a psychiatrist at Manchester Metropolitan University, told BBC News a sufferer would be "very selfish".
They would also have "a sense of entitlement, in that they expect praise and things to be given to them as it were, and to be treated almost as royalty, most of the time, by most people".
Marjorie Wallace, chief executive of the mental health charity SANE, said: "Although narcissistic personality disorder is not that rare, it is seldom that it becomes a pathological disorder that could lead to this kind of brutal act.
"With this condition there is a lack of empathy for other people, and is very difficult to predict or prevent a tragedy such as this."
Gillian Bloxham, a chartered clinical psychologist working in one of the treatment programmes for people with severe personality disorders, said less than 1% of the general population were thought to have the disorder.
Psychologists check for it with a test known as the Structured Clinical Interview for DSM-4 - Personality Disorders (SCID-4).
"It's not so much a test... it's more a case of sitting down with the patient or client and asking them about their life," she said.
"If they have a grandiose sense of self they tend to talk about best thing they've done or inflate small achievements."
For example, she said, a sufferer might say that they have medical experience, when in fact they once talked to a neighbour's daughter about an eating disorder.
"They tend to exaggerate in an immensely obvious way - as people they're unusual in their personality. It becomes very evident when a person is narcissistic."
Ms Bloxham said sufferers of narcissistic personality disorder really believed their own fantasies, meaning it could be hard to treat them.
"Why would someone who thinks they're special and great come for therapy?"
In her own experience, people started coming in their 40s and 50s when they realised they had not achieved their goals or were having problems in their relationships.
Personality disorders expert and consultant forensic psychologist Kerry Daynes, from Manchester, said: "Many psychopaths have narcissistic traits, and you most often see NPD when treating psychopaths.
"That is not to say that everyone with NPD is a psychopath, or that every psychopath is narcissistic - but they go together in a lot of cases."
Thinking themselves "above the rules that the rest of us work to", sufferers committed extreme, cynical and violent crimes to satisfy their needs, Ms Daynes added.
With "a great feeling of entitlement", they felt entitled to other people's respect, admiration and money and could be "very callous", Ms Daynes added.
She said: "It is good to think highly of yourself - but for these people it is out of control. It has gone off the scale."
The Mental Health Foundation website says at least three of the symptoms of the disorder need to be present for diagnosis.
"Like other personality disorders, narcissistic personality disorder is a controversial diagnosis," it adds.
"At present there is no consensus as to its causes or treatment."