I have been thinking on this and what I have come up with is this.

What is the likelihood that something happened suddenly, like a loss of cabin pressure, or more likely a slow loss of pressure or even the air supply being contaminated by CO or other gas that caused the Pilots to lose conciseness either rapidly or gradually and before they both loss conciseness they realized something was wrong and the first thing they did was set the flight computer to return to Malaysia and before they could transmit a distress call they lost conciseness.

Now the first thing they should have done was to go on O2 but what if the Pilots Emergency O2 System didn't work or was also contaminated.

Also what if the aircraft life support systems were sabotaged before the flight.

There are a few things that don't make any sense at least to me.

One is if one of the Pilots wanted to kill himself and everyone on board, why not just crash into the ocean shortly after takeoff instead of disabling the Transponder and Flight Data Transmitter and making a U-turn to pass over Malaysia.

Also the location where debris was possibly found is in the general area where the flight would have exhausted it's fuel supply.

One more thing, has there been any investigations on if any of the flight crew or passengers could have been the target of a murder plot?