In the interview with Nik Huzlan, Nik is clear that the failure to contact HCM within a fraction of a second of handoff from KUL was the key that whomever was flying did a deliberate action right at that point.
An alternate possibility is that a catastrophe hit right at that instant. Perhaps an oxygen bottle exploded at that moment piercing the hull. Perhaps a fragment took out a cable that fed ACARS, a small lingering fire took 90 seconds to take out the transponder.
Perhaps the Captain was only able to get as far as turning around for an emergency airfield and entering some incorrect numbers that resulted in the two turns and then a fixed magnetic compass course from then on.
The aircraft porpoised in without coming apart, sank without wreckage.