It is not often that one feels profoundly sorry for a young man who has bludgeoned his loving parents to death, taken their credit cards on a spending spree in America and then pretended they were away in Spain while their bodies rotted for weeks in his childhood home. All the same I did feel very sorry last week for 19-year-old Brian Blackwell, who was convicted of this terrible crime on Wednesday. He sobbed uncontrollably in the dock and wrote a miserable statement about how he missed his parents, how he longed to turn back the clock and be a child again.

There is something entirely pathetic about him. He is an exceptionally intelligent boy, who left school with glittering prospects as a medical student — yet his first weirdly revealing question to the police on his arrest was whether it would be cold in jail. It emerged in court that he is a habitual fabulist and liar with a weak grip on reality and a determination to live out some of his fantasies. All this quite obviously adds up to someone with something seriously, mysteriously wrong with him.

Not long ago he would have been condemned in the tabloid newspapers and probably in the courts as an evil monster. Today justice is beginning to be more merciful and the judge in this case accepted that this wretched boy, although not insane, suffers from acute narcissistic personality disorder and therefore could not be charged with murder. He was able to plead guilty to manslaughter on the grounds of diminished responsibility. Presumably this makes little practical difference. He will be locked up indefinitely and rightly so. But the story raises some extremely troubling wider questions about moral responsibility.

It must be right to make allowances for people with diminished responsibility or with mental illnesses, as the courts do and as we all do. But recently it has begun to seem that the notion of personal responsibility is being eroded by medical diagnoses. It is being medicalised. Indeed, personality itself is being medicalised, in the sense of having supposedly scientific labels attached to it. Narcissistic personality disorder of the sort that Blackwell suffers from is just one form of borderline personality disorder — a concept only about 30 years old.

Oliver James, the medical psychologist, claims that about 80% of convicted prisoners suffer from a personality disorder and most of them from more than one. That may be true — although I am sceptical — but it has some troubling implications. Why, for instance, were Jeffrey Archer and Jonathan Aitken so heavily punished and morally denounced if they were merely the victims of their own personality disorders and if they are not to blame for their startlingly weak grasp on reality? Why is anybody punished in prison for behaviour beyond his control? Equally one is tempted to wonder what the status of these personality disorders really is. What is the science behind them? I suspect that borderline personality disorder lies very much in the eye of the beholder. The medical and quasi-medical practitioners concerned do not agree on the causes or the criteria of such disorders nor on whether they exclude or include psychopathic tendencies. The latitude in all this is so great as to be distinctly unscientific. There are no objective measurements. Experts do not even seem to agree whether personality disorders are treatable.

In truth the distinguishing characteristics of one or other of these disorders would apply to some degree to almost everyone. If I found myself in the dock for any crime, I would certainly plead a personality disorder in mitigation and I am confident that I would qualify. That does not mean that personality disorder doesn’t exist, but it does mean that while there isn’t much scientific method involved, one ought to be cautious about the use of such labels.

Instead, however, we are getting freer and freer with exculpatory labels. While once children were called stupid, lazy, naughty or obstinate, now we have many syndromes and disorders — all still imperfectly understood — that medicalise their behaviour. We have attention deficit disorder, dyslexia, dyspraxia or fragile X and now — currently fashionable — autism and obsessive-compulsive disorder. If my child or yours is wilfully untidy, lazy or disruptive, there is often a genuine case for saying that it is not really his fault. In ordinary life this can be very helpful.

For years the behaviour of some of my nearest and dearest and the destructive choices they make has been a mystery to me and sometimes a painful mystery. (No doubt some of them would say the same about me.) I’m thinking of the charming and loveable person who endlessly finds new ways to mess up her life and alienate her friends. I’m thinking of the serial wrecker of relationships. I’m thinking, too, of the person whose weird little compulsions drive him and his relations almost mad with frustration. Then there is the employer whose coldness and real cruelties are something he himself cannot understand.

The fairly recent idea that these people are not choosing to behave as they do but are driven by conditions outside their control — which may well be largely biologically determined — can be an immense relief. It can bring understanding and acceptance — although not, of course, in the case of brutal murder. But how far is it practical or even acceptable to make such allowances in the real world when decisions, and judgments, have to made regardless?I think it is highly likely that the biological sciences will in time replace our primitive understanding of personality and responsibility with something much more sophisticated. The subtle interaction between nature and nurture will probably become much better understood, not least the physical changes to the brain of early traumatic experience and the environmental triggering of inherited predisposition.

Western culture is based on an idea of an integrated, coherent, solid-state self and on the related idea that we are all equally morally responsible. Yet now that science is gradually displacing these central ideas and pushing back the boundaries of responsibility and of normality itself, we are left with a growing hole in the centre of our moral universe; it is becoming harder and harder to believe in the necessary myth of equal individual autonomy. That’s what is most disturbing about this brutal murder.

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