Most civil aircraft reach top of climb approximately 30 minutes after take off, give or take. (Depending on ATC restrictions A/C weight etc)
This aircraft had, apparently, been airborne for a couple of hours, so well into the cruise.
From my experience out of WMKL you are usually pretty close to TOC as you coast out from the east coast of the Malaysian peninsula, depending on the hand over from KL to Singapore ATC.
These guys are quite good and you normally get your requested/filed level shortly after crossing the coast.
This, undoubtedly, sounds like a massive and rapid catastrophic disaster which completely overcame the crew before they could even announce anything on the radio.
Bomb, airframe failure, oxygen, cockpit incursion, meteor strike, something highly, highly unusual for this modern aircraft to simply fall out of the sky and disappear?