The 49-year-old nurse was also convicted of trying to poison 20 other patients during what police called a reign of terror at Stepping Hill Hospital in Stockport, 200 miles (320 kilometers) northwest of London.
In a note found by police, Victorino Chua described himself as "an angel turned into an evil person."
The father of two wrote "there's a devil in me" and that there were certain things he would "take to the grave."
Chua was found to have injected insulin into saline bags and ampules. Other nurses who were not aware of the tampering then used the bags and ampules, leading to insulin overdoses.
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The nurse was cleared of a third murder charge but convicted of attempting to cause grievous bodily harm to that patient. Other patients recovered after receiving similar insulin overdoses.
Hospital officials first contacted police in July 2011 after finding contaminated vials of saline and "unexplained" low blood sugar levels in some patients.
After finding the vials contained high levels of insulin, police launched a formal investigation.
Manchester Police Chief Peter Fahy today apologized to Leighton for the time she spent in jail after she was charged.
He said Chua seemed unfazed by her imprisonment: "He was quite happy to stand by while she suffered the consequences." One of Chua's victims was a 44-year-old multiple sclerosis patient being treated for a mild chest infection, who was pronounced dead eight hours later after being treated with a saline ampule containing large amounts of insulin.