The only thing I can think of is that he had some serious problems in his personal life (perhaps society pressure from the "disgraceful" pictures of the girls in the cockpit or whatever) and wanted to give his family a shameless "out". If he intentionally crashed out in the ocean, it's going to take forever and a day to find that airplane & try to piece together what really happened from the black box. I mean, it took like two years to find Air France and we had more tracking data for that, right? Now you're into a search field half the size of the United States, good luck finding it out there!
I'd like to believe that it landed somewhere and they were held hostage, but the coordination on that would be absolutely unbelievable, especially given how much communications technology people have. Even if they stripped everyone from their cell phones, people still leave cell phones in their bags & stuff, which could be pinpointed using GPS technology.
I'm guessing that the co-pilot took out the pilot & turned off the transponders, but continued flying for hours (into the middle of nowhere) without alerting the passengers, and then nosedived into the ocean. That way no one would suspect what was happening (it all looks the same from the air) and no one would have a chance to make an emergency call before they crashed (and would also be far out of range of any cell phone towers, especially if the co-pilot turned off the satellite communications systems).
I'm assuming it was either the pilot or co-pilot took the action, because not one person onboard made an emergency distress call, so they probably never suspected what was going on. And again, if it had been a systems problem with the airplane itself, why turn off the ACARS and then turn off the transponder, and THEN turn off course, and then keep flying for hours?