Published Wednesday, 21 March 2012
Mr Maginnis was 20 at the time of the killings. (© UTV)
The Prison Ombudsman said Mark Charles Maginnis, from Co Armagh, had failed to attend medical appointments.
This, along with his "poor management" of his type 1 diabetes - a condition he was diagnosed with aged nine - had a detrimental effect on the health of his feet.
Mr Maginnis' death in October 2010 after a sudden deterioration in his condition was consistent with septicaemia due to infection of foot ulcers, the report says.
The 40-year-old once refused to provide a drug sample and self-harmed on occasions, and at times he suffered from low mood and anxiety and worried about the long-term consequences of his illness, the report said. He overdosed in March 2007.
Pauline McCabe, the Ombudsman, said his condition had been "carefully monitored" by Maghaberry's healthcare team, however the complications arose from Mr Maginnis' "non compliance with his treatment programme".
She said: "The investigation found that Mr Maginnis' failure to attend some medical appointments, along with his poor management of his diabetes, regrettably had a detrimental affect on the health of his feet.
"It appears that Mr Maginnis' condition deteriorated very rapidly on the morning of 25 October 2010 and his death, occurring as it did, was not expected.
"There were quite complex reasons why Mr Maginnis may not have cooperated with his care, one of which was that after more than 19 years in prison, he found the outside world quite daunting when he needed to attend hospital.
"He also suffered, at times, from low mood and anxiety and was fearful of the long term consequences of his illness."
The Portadown man was jailed for life in 1992 for the murders of his 47-year-old father, RUC Constable Alfred Maginnis, and the murders of his lover Sandra Brown and her husband Jeffrey on 30 April 1991.
He was 20 at the time he shot the victims.
During his trial the court was told that he had carried out the murders because he had heard from his father that his lover Mrs Brown had made an offensive remark about him.
The Ombudsman's report will be examined by the Northern Ireland Prison Service and the South Eastern Health and Social Care Trust.
Ms McCabe added: "Whilst Mr Maginnis overall care was found to be consistent with that he would have received in the community, a small number of concerns were identified, particularly in respect of the Prison Service's response to his difficulty in providing samples for drugs testing for medical reasons."
"These issues of concern must be now be addressed by the Northern Ireland Prison Service and the South Eastern Health and Social Care Trust as part of their programmes for change."