Appears to be a pretty open/shut case this one. Depressed pilot, locks the other one out, crashes the plane. There have been a few recent examples (EgyptAir 990, although the Egyptians still deny that was pilot suicide). Fully expect a change of regulations, should have happened sooner though, I believe in the US two people have to be in the cockpit at all times.
Suspected from the beginning it was something sinister, it would be highly unlikely for an A320 to develop a huge fault that makes it fall from the sky after 30+ years of service, especially with its maintenance record. It would have already been found.