What would you do as pilot of a 777 (or comparable aircraft) if faced with a 'significant' electrical fire in the exact position south of Vietnam where MH370 deviated from course?
I am not a pilot yet but I am aware that in case of any in-flight emergency, the pilot rules are to aviate, navigate and then communicate.
This explains a lot about an electrical fire or such event, since in such cases, the first priority of a pilot would be to land the aircraft. Which thus explains the west turn towards Langkawi Airport, closest airport at the time, and also easiest to navigate to on auto-pilot since smooth terrain on that route from where they were (as compared to going back to KLCC).
Second priority would be to find the source of the fire, i.e. assess the problem. In case of electrical fire, they would switch off electrical "busses", and turning them back on one by one to isolate where the problem comes from. Thus explaining lost of ACARS and all comms.
Climbing to from FL350 to FL450 would explain going to highest flyable altitude to minimize oxygen in an attempt to switch off fire. However, very difficult not to stall at this altitude, which explains sudden drop to FL250, a/c may have stalled and pilots struggled to regain control which happened at FL250. Heading continued on auto-pilot to Langkawi airport.
Lastly would be to communicate, but I guess things happened quickly, and since "busses" were off, that did not happen as something probably incapacitated the pilots (smoke for example), and they fell unconscious. Thus flight continued on auto heading until fire destroyed control electricals, or they ran out of fuel, thus crashed.