Gareth Williams: Take Off Your Tinfoil Hat

"For people who feel a need to believe in conspiracies, here's one sitting there waiting for them. Just imagine the CIA deciding, How can we undermine and destroy all of these popular movements? Let's send them off on some crazy wild goose chase which is going to involve them in extremely detailed microanalysis and discussion of things that don't matter. That'll shut them up. That's happening... In case anybody misunderstands, I don't believe this for one moment, but it's the kind of thing that goes around."

Noam Chomsky.

Of course, the open verdict in Gareth Williams' inquest is an invitation to stupidity and wild speculation. 
Otherwise intelligent people lose all power of reason.
How depressing.
Williams was involved in quite mundane deskwork - certainly not any James Bond subterfuge and derring-do
The secret services would have wanted to make sure no sensitive and/or confidential information was exposed, this would be standard procedure, and is not indicative of any sinister intent.
There was no evidence of foul play in Gareth Williams' death.
It's beyond daft to then argue; "Well, someone must have destroyed the evidence, then."
It's far more likely that there was no evidence, because there was no evidence.
Just as the reason there is no evidence that monkeys built the moon out of cheese in 1949 is not because someone has destroyed the evidence, but because there is no evidence for it.
Just as it's far more likely Gareth Williams had an unconventional sex life, rather than him being taken out in a hit by Blofeld and his hired goons at SMERSH via the notorious "Put a body in a sports holdall" assassination technique.
I'm sorry to be the one to break it to you, but real life and the real world tend to be rather boring and mundane.