Pilot & CFI here, even so, this is still (hopefully more than half-assed) conjecture
i like chris goodfellow's
wired article because it covers the most bases in a rational, linear way.
doubt there's foul play. top clue: when terrorists bring down jets, they'd LOVE to BRAG about it. no one has stepped forward...yet.
things he didn't emphasize enough/neglected:
assuming the fire flared after the last radio call, the first thing to do will be to put out the fire hence the fuses get pulled (along with transponder, radios, etc..)
the first turn to langkawi is sensible as it's towards the nearest long runway. after that, suppose the smoke and fire extinguisher chemicals get into everything around the cockpit and other systems. things go out in some seemingly random pattern. eventually every major system (like navigation) has fried/failed.
maybe the captain was the one mortally wounded in the fire. he tells the copilot what to do and dies.
so now you have a copilot fucking glad he's flying a boeing (because those fucking fly-by-wire airbuses woulda long since died and dived) but privately shitting that he's
dead reckoning flying at 30,000+ft, on a half-moon night, over open water & featureless terrain from a point he forgot X minutes ago (so he can't calculate the X minutes ahead to get to langkawi)
so copilot follows orders and guesses on time to field and when time's up he looks down and sees only open water. no land. time now is 2-4am. no light. starts freaking and drops altitude to look for shoreline. (or he stays at altitude -- who knows? -- the point now is he's a way too raw pilot is flying the jet.) this dude is SCARED.
problem with that is that by going lower he increases fuel consumption (reducing speed and range), reduces the distance he can see and kills his fuel out glide range. starts making guessed turns hoping for land. still no dice in the middle of the night.
maybe by 5am enough light peeks out so he decides to fly east. (by climbing or descending) he burns more fuel and kills his range. fuel out. splash.
i dunno if they'll ever find the plane but i think last known track of the plane was eastward toward the sun. but given that flying reg's are, as we say in the biz, written with blood, i'd bet that there'll be a new requirement that all overwater planes carry a handheld, battery-powered GPS device (in case everything fries) like fire extinguishers.
one of the things that older pilots harp endlessly on about modern cockpits is that it turns pilots into button-punching idiots with no handflying or basic pilotage sense. if you disable the systems on a modern jet, most overseas pilots -- especially those fast expanding airline cadets, i believe, will freeze up -- just like i think this copilot did.
i currently know of ZERO airlines that require backup, independent gps units being carried aboard. this is something that should happen.