Lingering, conscious, suicide is an extremely unlikely guess.
Maybe after decompressing the cabin section to kill all the passengers and crew, the suicidal pilot (or co-pilot) set a new course for the most remote ocean area within the plane's fuel range and then chomped down on a cyanide capsule or took something that would knock him out so the end would come without his being awake...
Maybe he set this course and had the plane fly for six more hours because even as deranged as he was, on some level he felt guilty about what he was doing and was trying to make the plane "disappear" in area where it would be least likely to be found, and thus his actions least likely to be discovered...
Maybe he kept the plane flying because he had some bizarre ritual he wanted to perform...
Maybe he kept the plane flying because he got cold feet and couldn't bring himself to put the plane in a nose dive, so he just let nature take its course...
All of those things are possible...
It's also possible that the pilots were displaced by a passenger or passengers, (maybe the reason there's never been a claim of responsibility is because they were just a couple of nutcases acting alone) who had similar motivations, (or who wanted to "terrorize" by creating a mystery; whether they were free lancers or part of a group) ...
All of those things are possible; given the known facts...
Though I have to say that the "terrorist" theory, while not ruled out by the known facts, seems somewhat weaker to me; if you apply
Occam's Razor:
For anyone other than either the pilot or co-pilot to have done this, you have to posit a struggle, and then you have to posit hijackers who had knowledge of the plane, its systems and operation equal to that of pilots experienced at flying the plane...
If it was one of the pilots, you don't need any of that to explain what happened; they already controlled the plane, and we know they had the expertise...
I agree that for a person or persons bent on a suicide mission with an aircraft to keep it aloft for six hours is "extremely unlikely"...
The problem is, that if the plane really went down where all the best evidence indicates it did, (of course if it
didn't go down in the Southern Indian Ocean, then
everything is back on the table; accident, Doomsday Plane, alien abduction, etc.) as "extremely unlikely" as that explanation may be, it's the only one that's possible...
It
couldn't have gotten there by accident, (because
somebody had to make that second course change) and it
couldn't have gotten there with anyone intending to land it...( because there's no place to land it there...)
One further point:
I'm not particularly impressed with the fact that thus far the investigations into the pilot and co-pilot's lives haven't turned up anything that would be some sort of a red flag that something like this might have been in the offing, (especially now that we pretty much know for a fact that this must have been a suicide mission on
somebody's part)
There are plenty of examples of people who have killed their own families (and also after that frequently themselves) who weren't experiencing financial or interpersonal problems or who appeared to anyone else to be under any sort of duress...
They just lost out in a struggle with their own private demons; a struggle unknown to anyone but themselves...
That may very well be what happened here...