Thank you Starlionblue. So, we can say, theoretically, any problem with the aircraft (unless hypoxia/absence of pilots or complete loss of control), the plane would not have left the area. Am I correct?
If we take the first one, I don't think hypoxia happens in a second. It'll take a while and crew might have had some time to respond (whether they know they are going into hypoxia or not). If they know about loss of oxygen, they would have reduced the altitude and continue to address the problem they were facing before. Even if they didn't, they anyway turned (assuming they tried to return/land). The descent would have solved the hypoxia problem, right? Unless there was no descent at all and it was not their intention to do so.
That leaves us, only one situation. Complete loss of control. If not, we can assume, this is where all other theories start.
1. Absence of pilots - physically in the cockpit or intentionally with the controls (they were just sitting back with suicidal intentions).
2. Hijacking - Whether by pilots or a passenger. Assuming passengers, the pilot bravely tried to take the plane away as far as he could go, so he can avert another 9/11.
In both cases, I'm sure the pilot cannot be suspected. Psychologically, a person with suicidal intentions will definitely leave a message. At least to his loved ones if not to the world. We have no leads on this.
If Pilot is suspected with the hijacking, a first time hijacker cannot be this perfect. And there's no way in the world he can handle this alone without support from a group/nation.
It's far easy to do a background check on the pilot. His psychological or social activities in the recent past will tell something. Remember, we are not talking about an expert terrorist but a first timer according to the facts we have. So there would be indications about a major change.
And this is where I'll definitely conclude "It's just an accident". Nothing more, nothing less.
@ Experts: Don't BS
me. It's just one of the many angles we could think.