What kind of person could kill Holly Wells and Jessica Chapman? John Wayne Gacy, who murdered 32 young boys during the 1970s, said: "I see myself more as the victim than as a perpetrator. I was cheated out of my childhood. If only there was someone, somewhere, who could understand how badly it hurt to be John Wayne Gacy!"

Gacy perfectly expressed a position that has come to hold the status of orthodoxy in the eyes of the criminal justice establishment. No one is essentially, irredeemably bad: they are made bad by circumstances, and their badness can be cured if their circumstances are made better.

The worse the crime, the less likely it is that the perpetrator was fully responsible for it. He must have been in some way mentally deranged, either by his experiences or by a mental illness, to be capable of it: a reasonable person could not have done it.

That attitude has become part of our system for dealing with very serious offenders. The essential feature of that system can be summed up as "compassion towards the offender". Hate the sin, but love the sinner. He too is a "victim": a victim of social forces, of poverty, of an unhappy childhood, of mental illness, or whatever misfortunes can be discovered in his situation.

Increasingly, the most serious criminals - the sort of people who are capable of the murder of Holly and Jessica - have not been seen as suitable for the criminal justice system, which requires that those accused be judged fully responsible for what they do.

They are seen rather as cases for psychiatrists, for "medicine" - an attitude that has not merely been reinforced, but greatly extended, by the Government's latest proposals for changing the 1983 Mental Health Act.

The effect of those proposals is to broaden the notion of "mental disorder" so widely as to ensure that many of those suspected of being capable of serious crimes could end up being taken out of the orbit of the criminal justice system altogether; their liberty would instead be solely at the discretion of psychiatrists assessing whether or not they had successfully responded to "treatment". The most heinous wickedness would be officially re-categorised as a kind of illness.

Underlying the belief that very serious offences are really a medical rather than a moral problem is the optimistic conviction that only the mad are capable of truly dreadful crimes. That optimistic conviction is, however, profoundly mistaken. The depressing truth is that people do not need to be mad to be capable of crimes such as the murder of Holly and Jessica. They can be sane, rational, and totally unreformable by any psychiatric technique - or indeed any technique at all.

An increasingly weighty body of expert opinion now recognises - contrary to orthodoxy - that there are people whose only defect is that they are horrible. They know when they hurt others - they just don't care.

The notebooks of Norman Russell Sjonborg, an American lawyer who murdered one of his clients, showed that he was, to quote his horrified probation officer, a "man who could contemplate killing his own children with the detachment of someone considering various car insurance policies".

"Do I feel bad when I hurt someone?" another rapist and kidnapper is recorded as telling a psychiatrist. "Yeah, sometimes. But mostly it's just like. . . uh. . . (he laughs). I mean, how did you feel the last time you squashed a fly?"

Such people are not ill. They are not mad. They simply lack the moral faculties which the vast majority of us take for granted. Their salient characteristic is that they are phenomenally selfish. They lack any capacity for empathising with others. They can think with brutal logic about how to get what they want, and then act so as to achieve it - simply ignoring the harm that they do to others on the way.

They can be charming when it suits their interests, and they are brilliantly effective at manipulating others. They are not fragile individuals. They have a hard, tough, solid personality structure that is extremely resistant to outside influence. Indeed, they have proved themselves masters at manipulating those required to assess and "change" them.

Dr Robert Hare, an academic who worked for several years inside a prison, describes how he discovered that "prison records were filled with carefully written personality profiles that seemed embarrassingly at odds with what everyone knew about the inmates in question. I recall one file in which a psychologist had used a battery of self report tests to conclude that a callous killer was actually a sensitive, caring individual who only needed the psychological equivalent of a warm hug."

The psychologist's attitude is contemptible, but it is in fact only an extreme version of a view that is widespread in the criminal justice system. To take one recent example: Paul Beart was convicted in 1998 of a brutal sexual assault. He was sentenced to five years for it.

He quickly persuaded prison officials who wanted to treat him that he was suitable for a rehabilitation course. He seemed to complete the course successfully and so was released two years early. Within months of that release last year, Beart sexually assaulted and murdered Deborah O'Sullivan, a woman whom he picked out at random.

Beart has now been recategorised as an "untreatable sexual sadist". It is unfortunate - to put it mildly - that it took so long for the professionals to realise this. Yet the Government seems determined not just to cling to, but to extend, the idea that criminal behaviour like Beart's is best assessed and judged by medical professionals.

The more rational psychiatrists do not want the power that the Government's proposed changes to the 1983 Mental Health would give them; they know that they cannot tell accurately who is dangerous and who is not, because they know they can be as effectively manipulated by people with "personality disorders" as the rest of us.

"These people do not belong in hospital. There is nothing we can do for them; we can't treat them and we can't change them," says Dr Anthony Zigmond, a consultant who is leading the Royal College of Psychiatrists' concerted opposition to the changes to the Mental Health Act.

The Government's reforms are intended to prevent terrible crimes. They are doomed to fail. In most cases, the people who kill children do not suffer from medical problems at all. The cause of the child murderer's actions is not his circumstances or his condition but himself: he is irredeemably evil.

The institutions for dealing with such people need to be based on recognising that truth - not on the fantasy that psychiatrists can identify in advance who suffers from a dangerous medical condition and who does not.