Smoke in the cockpit area would convince me that the pilots were not the villains.
It is either a design/maintenance problem or sabotage.
I suspect the latter.
No way was this a mechanical malfunction. If you study the flight path of this aircraft it went down in the worst possible place for finding the fuselage (open ocean and the ONLY place along the entire flight path where the ocean was the deepest). This is NO coincidence and defies statistical probability for it being a mere malfunction.