Fly26 we seem to have had the same thought processes!
My guess?
One or other pilot had a 'spectacular' in mind, as you suggest, to let the world know of his grievance perhaps (I won't speculate here what those were, but I have my suspicions).
So this person acts at the ideal moment - takes control - disables anyone else on the FD - then gets on O2, switches off the pax O2 and depressurises the cabin (outflow valves open). The reported climb to FL400(?) was to guarentee total final incapacitation of all others on board.
Aircraft is then descended and turned. This is where the plan changes. I think something changed his mind and he snaps out of it. Instead of the 'spectacular' he slinks off to die in the empty and vast Southern Ocean.
That's all speculation of course.
But having taken the a/c succesfully, in such a planned way, was this really the planned endgame? Or an improvisation?