What I think happened is that there was probably a malfunction in the area of the plane that holds all the wiring and electrical stuff for the technology that controls it; possibly a fire that caused damage to other things, such as the AKARS system. The head pilot probably felt obligated as the one with more experience to go down and check. Perhaps the fire caused fumes that were toxic rendering the pilot incapacitated and the co pilot (who looks younger than me) probably shit his pants, since he was flying over the ocean, too far from anyone for verbal communication, and with the AKARS system malfunctioned. The sharp turn was most likely him trying to turn around and head home, but he probably got confused, and just flew until the plane ran out of fuel.
Honestly, I feel like it was most likely just some kind of technical failure. If the debris they found is the plane, which I'm doubting the more I think of it, and they can retrieve the "black box" hopefully we'll know more. I just feel like the simplest answer is usually it.