Nurse Victorino Chua, 49, injected insulin into saline bags and ampoules while working on two wards at Stepping Hill Hospital in Stockport in June and July 2011.
These were then unwittingly used by other nurses on the ward — leading to a series of insulin overdoses to mainly elderly victims.
Chua was convicted of murdering two patients but cleared of a third murder by a jury at Manchester Crown Court, which had been deliberating for 11 days.
Prosecutors said the Filipino father of two had decided to take out his personal frustrations on patients “for reasons truly known only to himself”.
After police were called in, Chua was said to have “changed tack” by sabotaging prescription charts, doubling and trebling dosages — some with potentially lethal consequences — leading to his arrest in January 2012.
Among the evidence produced by the prosecution was a self-penned letter found at Chua’s home in Stockport after his arrest.
In the letter, described as “the bitter nurse confession” by Chua, he said he was “an angel turned into an evil person”, that “there’s a devil in me”, and how he would “take to the grave”.
He was found guilty of murdering Tracy Arden, 44, and Alfred Weaver, 83. He was cleared of murdering Arnold Lancaster, 81, who was suffering from cancer, but convicted of attempting to cause him grievous bodily harm with intent by poison.
Arden, who had multiple sclerosis, was admitted for a “mild” chest infection and would have expected to “sail through this storm”. However, she was pronounced dead eight hours after admission after being treated with a saline ampoule contaminated with insulin.
Weaver was admitted with a chest infection and, after being given a saline drip, he “appeared to be in agony, eyes rolling back in his head”. He died 10 days later.
Chua will be sentenced today.
Prosecutor Peter Wright said the “bitter nurse confession” was “a living and unfinished document”.
Wright said: “There was unfinished business. It is a narrative of his life, of his feelings, of his pent-up frustrations. It was a portent of things to come and of what he had done.
“It is an insight into his thought process during a period of considerable anger and disharmony both at home and at work which coincided with these events.
“The evidence points sadly to a man who, for reasons truly known only to himself, decided to take out his frustrations on his and others’ patients.”
Chua’s defence team said he had been wrongly singled out and made a scapegoat.
Counsel Peter Griffiths said the effort that Greater Manchester Police had put into the investigation resulted in “huge pressure” to bring someone to account.
The defence reminded the jury that none of Chua’s fingerprints were found on any of the contaminated ampoules or saline bags — and no one had seen him sabotaging any products while working at the hospital.