The current 'official' operational theory is this plane was, for lack of a better word, 'stolen' by person or persons unknown who knew what they were doing (disable transponder, ACARS, etc., took flight path to avoid radar detection) and who had meticulously planned the operation in advance. A turn west from/after IGARI was entered into the active flight plan in the FMS; this is known as the last ACARS report indicated this waypoint change event.
So, how come the highly knowledgeable careful planning 'perp(s)' left this big fat clue? -yes, perhaps the were unaware ACARS would report a waypoint change event however if one has carefully planned this type of operation one would certainly not do anything out of the ordinary before disabling ACARS.
So, again the Q