According to Corona Dr. Fiona Wilcox, Williams was probably murdered -- quite possibly by his own employer, the British Government.
Williams worked for MI6 who, according to his family's solicitor, failed, following his disappearance, to make "... even the most basic inquiries about his whereabouts and welfare."
The Coroner concluded that:
Most of the fundamental questions in relation to how Gareth died remain unanswered.She said that evidence given by "G", Williams' MI6 line manager stretched "the bounds of credibility". She also recorded the fact that during the investigation Scotland Yard were not permitted to speak with Mr Williams' MI6 colleagues.
Dr. Wilcox stated that the lack of hand and footprints in the bathroom where Williams' body was found was "significant".
It was "highly unlikely" she said that Mr Williams got inside his red holdall alone:
If Gareth had been carrying out some kind of peculiar experiment, he wouldn't care if he left any foot or fingerprints.Mr Williams' family told the inquest that they believed Gareth was likely murdered by an agent of the security services.
For a much more comprehensive review of the case see Winter Patriot's: Sherlock Holmes and the Alderney Street Mystery.
50 agents face DNA tests over spy-in-the-bag killing as coroner brands death 'unlawful' and puts MI6 in the frame.