Serial killer and rapist Sinclair died on 11 March aged 73 at HMP Glenochil in Alloa.
He had spent more than half of his life behind bars for a string of brutal killings and sex attacks on girls and young children.
His despicable list of crimes included the 1977 rape and murder of Edinburgh teenagers Christine Eadie and Helen Scott.
The shocking case became widely-known as the ‘World’s End Murders’ as pals Christine and Helen had last been seen together at the World’s End pub on the Capital’s Royal Mile.
The World’s End murders was for decades one of Scotland’s highest-profile unsolved crimes, until 2014, when Sinclair, who had been locked up since 1982, received a 37-year sentence.
Now it has been revealed by the Scottish Sun that Sinclair died from pneumonia and heart disease.
With no relatives willing to come forward, his death registration was carried out by Police Scotland.
The post-mortem explains Sinclair met his end because of bronchopneumonia, which can cause severe inflammation of the lungs, and ischemic heart disease, that can bring on angina - ultimately leading to a heart attack.
Battling ill health for the past five years, Sinclair was described an “empty shell” after suffering several strokes.
The twisted killer spent his final days with an emergency button strapped to his wrist in case he needed to alert guards in the event of a medical emergency.
It is understood Sinclair had recently suffered a series of fresh mini-strokes in the lead up to his death and had requested that he not be resuscitated if he collapsed again.
He was pronounced dead at 4.20am on 11 March, with his death certificate describing him as a ‘painter and decorator’, an occupation he would’ve been unable to undertake in any official capacity since the early eighties.
Sign up to our newsletter: enter your email in the box at the top of this article to get daily updates straight to your inbox.