Assuming current information is correct regarding the PNF's re-entry to the cockpit, some serious questions will now need to be raised as to how this security measure after 911 has royally backfired.
Like in the US, if one of the two pilots needs to leave the cockpit, a crew-member should take their place until said pilot returns.
There should never be a situation where one pilot gets left in there with the capability to prevent the other returning.
It could be as simple as a heart attack surely, rather than a suicidal or terrorist pilot?
What a damn shame...