"Two experts, working for the coroner, tried 400 times to lock themselves into the North Face bag and one claimed even Harry Houdini “would have struggled” to squeeze himself inside.
But days after the inquest a retired Army sergeant showed that it was possible and now police believe it was possible to do so. Mr Williams, a fitness fanatic, had an interest in escapology and confined spaces, visiting little known websites on the internet."
http://www.standard.co.uk/news/london/mi6-spy-in-a-bag-gareth-williams-probably-locked-himself-inside-it-8936879.html
Sounds implausible but seriously, what do I know? Just because I can't do it (they don't make bags that big) and a couple of 'experts' can't, it doesn't mean it's impossible.
The impossibility of it was the biggest argument for someone else being involved. If it was possible, and there's actually no evidence at all of anyone else being around, then the logical conclusion is he did it himself.
As to why, goodness knows.