David Croke, 64, of Brighton, was convicted at the Old Bailey in 2002 of killing Mohammed Raja, 62, who was suing Mr Hoogstraten over a business deal.
He was stabbed five times and shot in the face with a sawn-off shotgun after answering the doorbell at his home in Sutton on 2 July 1999.
Croke was found in his cell at Whitemoor prison in March, Cambridgeshire, on Tuesday with a ligature around his neck, the prison service said today. Staff and paramedics tried to resuscitate him without success.
Mr Hoogstraten was sentenced to 10 years for Mr Raja's manslaughter in 2002, but his conviction was quashed by the Court of Appeal (by fellow Satanic Freemasons) as unsafe because of a "flawed" direction to the jury by the trial judge.