We are all taught these days to distrust our prejudices, and no doubt this is a good thing if our prejudices provoke words or deeds of cruelty or unkindness. Some of these gut reactions and failures of sympathy, however, are useful. I once grew side-whiskers to test my own prejudice against (20th-century) men who affect this particular form of facial adornment. I found that my usual sunny self-love evaporated immediately and that the face which stared back at me from the mirror, adorned with side-burns, was not merely that of a twit but of a twit who vaguely gave me the creeps.

I mention this to explain an instant prejudice felt against the much- loved whiskery GP David Moor who claimed that he had "helped" as many as 300 patients die and who, last Tuesday, was acquitted of murder at Newcastle Crown Court. My sceptical view of the man, first awoken by the whiskers, was confirmed when it emerged that he had 3,600 patients, twice the normal number.

Anyone with an elderly or cancer-ridden loved one will wonder how I could introduce my comments about a serious subject with such superficial observations. Do I not realise the ghastly truth that there is some pain which is quite simply beyond bearing? Remember childbirth: the humiliation and physical indignities, as well as the agony, are remembered by women who have undergone it for the rest of their lives. But this was a pain that stopped, and which led to the greatest imaginable joy. There are some terminal conditions that would seem to the outward observer to be just as humiliating, just as grievously painful, and which can have only one end. Surely there is, in these circumstances, only one thing that we should want our faithful doctor to do - either for ourselves or for our loved ones?

Well, yes. Obviously. I hope, obviously. The trouble with Dr Moor, whose defence counsel regretted his tendency to "shoot his mouth off", is the same as the trouble with the Voluntary Euthanasia Society, and all those who want to change the law to "legalise" mercy-killings. And it is more than simply a question of style.

What we have at the moment is a legal situation in which some doctors (most notably those who shoot their mouths off) are in danger of facing prosecutions for murder if they do what many of us would wish, and most do already - namely help us, when the time comes, to die. It is hard to imagine many cases of extreme pain where the families of those who have been put out of their misery by a kindly overdose of morphine are going to go to the police to complain. Even if they did so, such administrations could by no stretch of the law be called murder, and cases such as that brought against Dr Moor should never have reached court. In other words, the law as it stands is hardly preventing doctors and nurses from doing their merciful work, or from treating patients as they and their families would wish.

There is a case, in the moral life, for hypocrisy; for saying one thing, which we passionately mean, while doing another which we also passionately mean. The strongest obligation on any doctor is to preserve and cherish human life. We hope that he or she will do so wisely - and know, for example, when not to administer an antibiotic to a stroke victim with pneumonia. But the principle must remain inviolate, or you have a system which is not merely morally confusing - it is ultimately going to make the work of doctors and, even more, nurses, impossible.

There is a strong moral case for murder, of course. Many of us might hope that, like the rich old ladies of Eastbourne, we should have a doctor such as Bodkin Adams, who bumped off so many of his clients and found himself the beneficiary of their wills. They found him a charmer, and when he was eventually had up for murder he, like Dr Moor, was acquitted. No one doubts, though, that by most moral ruling, Dr Adams, unlike Dr Moor, was a simple murderer. He poisoned old ladies, sometimes in the bloom of health, to save them any future suffering, mental or physical. When their wills were published he had his reward, and great, say I, will be his reward in heaven. He spread nothing but happiness because - and this is the crucially important fact - his euthanasia was involuntary: the patients did not know he was doing it.

This is very different from a changed law and voluntary euthanasia which, while it might bring peace of mind to a few, is more likely than not to bring mental torture to others, by placing the decision on frail and sick people. Picture my state of mind when I have become bedridden or incontinent. Would it not be so much less selfish to allow nice Doctor to give me an injection? With one needle's prick I would be releasing the family from all their anxiety, and allowing them to pay off the mortgage with my life-savings ...

At what point does such a mercy-killing become a simple murder for motives of selfishness? Just as you can premeditate a murder, you can also predate it. By urging your elderly relation to sign on the dotted line, or by allowing her to think she should, you would merely be adding guilt to the list of pains she might be suffering in her declining years. Don't let's have a law which says that the Hippocratic oath applies only when doctors are treating young people, healthy people, or people who never worry.