Colin Campbell Norris got away with his first three murders between June and October 2002 because the deaths of the three elderly hospital patients in his care were put down to natural causes.

By the time the staff nurse turned his attention to Ethel Hall, a retired shopkeeper who had fractured a hip after a fall in her kitchen, Norris was so sure that he had perfected his deadly art that he felt bold enough to issue advance notice of the killing.

Mrs Hall, 86, was making a good recovery from her routine operation at Leeds General Infirmary (LGI) and was looking forward to going home, but when Norris started his November 20 night shift at the LGI, he predicted to a colleague that she would be dead before dawn.

Indeed, he went further. At midnight, he remarked to another nurse that someone always died when he was on nights. It was "always in the morning . . . about 5.15am", when "things go wrong", he said.

At 5am, Mrs Hall duly suffered a sudden deterioration in her health and fell into a coma in which the grandmother remained until her death three weeks later.

Many of the people who worked with Norris have found it hard to reconcile such cold-blooded acts with the man they knew. The dark-haired, dapper young Glaswegian is remembered by colleagues as "a nice friendly lad", a hard worker with a dry sense of humour who "wouldn't hurt a fly".

Norris was only 25 when he took a job as a staff nurse with the Leeds Teaching Hospitals Trust after graduating from Dundee University with a diploma in nursing. For 15 months, from October 2001 until his suspension in December 2002, he worked shifts on orthopaedic wards at the city's two big hospitals, Leeds General Infirmary and St James's.

But the one area of his work that Norris appeared to dislike was dealing with the elderly. Vulnerable, often confused, sometimes incontinent, such patients seem to have been regarded by the staff nurse as little more than an irksome burden.

He confided in a trainee nurse that he did not like old people, and some elderly patients recalled at his trial that he had been rude and dismissive to them.

To graduate from dislike to murder is a huge step, which may partly explain why it was not until Norris killed his fourth and final victim that the hospital authorities became suspicious.

Norris was said to have displayed "an air of detached amusement" at the unexplained collapse of an earlier victim, but such lofty disdain would not - for long - be possible on this occasion.

The doctor on duty was so concerned about Mrs Hall's inexplicable descent into a hypoglycaemic coma that he summoned the assistance of Dr Emma Ward, an endocrinologist and diabetes specialist.

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