From the Oban Times:
The plane was seen going down behind the hills at a steep angle by two separate hill walkers in the Ardmaddy area,who reported it to emergency services.
This appears to have been loss of control at altitude - not scud-running. As there is no fuel at Tiree, they would have had fuel to return, and probably divert to anywhere in Scotland. The decision by an IR(R)/IMC rated pilot to fly the route does not seem unreasonable.
TAF COR EGPU 041206Z 0412/0414 29009KT 9999 BKN015 TEMPO0412/0414 1200 DZ BR BKN002
Pilot error does not appear to me to be the most likely cause. Airframe break-up due to severe turbulence/overspeed?
Overspeed reulting from A/H failure in cloud? CO affecting occupants?
I assume they would have been able to recover from a spin in the VMC conditions where the hillwalkers saw them.