If I understand the timeline, the plane's last comm was leaving one freq for another. Given my experience with IFR flying, if I wanted to disappear with max time until someone noticed, that is when I would do it. Given the odds of random plane malfunctions right at that second, this seems to argue very strongly for human intervention by someone familiar with flying.
I'm not sure they were simply leaving one frequency for another - I thought they were going out of range of the Malaysian ATC and would not be in range of Vietnamese ATC for some time.
Granted that makes it an even better window for any bad guys, but that possibility is not set against the unlikelihood of a "random malfunction [...] right at that second". There's a longer window for problems to arise.
(But I freely admit to having rather lost track of what are the established "facts" in the timeline, between the various versions and corrections in official announcements, confusion over time zones, and the amount of recycling of rumour and supposition as fact that has been going on here from time to time...
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