My first reaction on hearing that doctors may be trying to blow us up was one of horror - how could people who have dedicated themselves to saving lives set out to take them? But this instinctive response is flawed. Medicine is a career like any other, one that attracts bad people as well as good, and it has always had a dark side.

How do you become a doctor? By practising dissection on human beings - by doing something that we instinctively recoil from in horror. What is surgery but cutting into living flesh? Every doctor has to learn to think of the human body in a detached way, has to train themselves not to be squeamish, has to be prepared to inflict pain.

This is less obvious in the modern world because we have anaesthetics, adopted in 1842. To be a surgeon before then was to learn to ignore your patients' howls of agony. The great 16th-century surgeon Ambroise Paré said that a surgeon must be "resolute and merciless". The phrase "first do no harm", commonly attributed to Hippocrates, was, in fact, not used until 1860, when inflicting pain had, for the most part, ceased to be part of the doctor's daily routine.

Doctors have power over their patients, and wherever you find power you find the temptation to abuse it. Often this is carefully hidden and displaced. Harold Shipman killed approximately 250 of his patients between 1971 and 1998. The deaths he inflicted were almost always painless, or so Shipman could have said; even as he killed, Shipman could tell himself he was merciful.

The fact that doctors might be drawn to death should not surprise us. Nor should we be shocked if people with good careers and incomes throw everything away for a murderous cause. It has been observed that 45% of doctors joined the Nazi party - a higher percentage than any other profession.

We have a tendency to assume that the roots of evil are sociological - deprivation, exclusion and prejudice generate hostility in return. But evil has psychological as well as sociological causes. Killing makes people feel important and powerful, and megalomania is as common among lawyers, accountants and teachers as among the deprived and the dispossessed. The power that doctors exercise over patients can tempt them into callous indifference or worse. Most, of course, resist this temptation. But some are bound to succumb.

· David Wootton is the author of Bad Medicine: Doctors Doing Harm Since Hippocrates.