Freeman was jailed in July 2011 for possessing indecent pictures, films and drawings of children, after more than 3000 harrowing drawings were found at his home south of London.
At London's Old Baily, he was handed an indeterminate term for public protection with a minimum of 30 months for possessing the "vile and disgusting" images.
So large was the collection of "vile and disgusting" child pornography found at Freeman's home, the court heard, that it was "among the worst" officers had ever seen.
Freeman led the Paedophile Information Exchange (PIE), a pro-paedophile activist group which campaigned openly for the abolition of the age of consent.
The Exchange - once described as "an international organisation of people who trade obscene material" - campaigned for "children's sexuality".
It wanted lawmakers to lower the consent age and aided adults who endured "legal difficulties concerning sexual acts with consenting 'under age' partners", the BBC reported.
He was serving an Imprisonment for Public Protection (IPP) sentence ahead of his death, meaning he was jailed indefinitely.
It is not known if he ever applied for parole after serving his minimum sentence of 30 months.