He was thought to be working on cracking Mossad's intelligence. This is at least the rumour on the conspiracy files. Maybe the British and Americans were trying to find out what exactly Mossad has on them.
Much of the speculation comes from the fact two Middle Eastern people came out of the flat of Gareth Williams shortly before he was found dead. Most suspect Mossad.
Other speculations abound that Gareth Williams knew of a Russian, former KGB mole operating inside MI6. He was about to expose this. So it is believed the Russians bumped him off.
Zionists at the helm of Britain, MI5, MI6 will sure to keep the truth of the matter from the British public.
Many suspect MI6 not Mossad behind the death. They point to the fact he was in a MI6 flat and they left it for a week before contacting him once he didn't turn up for work. A secret meeting between MI6 and the MET took place only adding to people's suspicions. Either way some rogue element finished him off. One interesting blog to look at is http://tswktm.blogspot.co.uk/