I don't think it is a simple case of suicide. He has good experience and knowledge but yet there was no distress signal. This led me to opine that he might have committed a silly error during flight and tried to correct it on his own without alerting ground control. However, one error led to another.
Until we find the aircraft and retrieve the CVR and FDR, we can only speculate on the cause but given what I've read about the Pilot and the safety record of the B777, I find it hard to believe the incident is caused by pilot error on the most popular medium to long range passenger aircraft in the world.
I'm sure the authorities and even the press have dug deep into the personal background of Capt Zahari Ahmad Shah and so far no-one has revealed anything close to controversial.
Compare him with Capt Tsu, the pilot of SilkAir 185. Now there's a man who had mental and psychological issues...gambling debts, tampering with black box circuit breakers, taking out an insurance policy one week before the incident etc. All circumstantial but as they say, there's no smoke without fire...