Lets say you were a pilot with intent to commit suicide (and take everyone with you; ignoring the sociopathy involved in that)... Why go through the effort of 'hiding' the plane? Turning off the transponder and comms, changing altitude and direction, and flying for a few more hours? The plane was already over the ocean, easy to dive it straight down. Less than a minute and it's done.
My intuition says that someone wanted a 777 and wanted to hide it.
So first, a potential argument against the suicide scenario is that it would be more rational to simply fly the plane straight down instead of flying on for 7 hours. Of course, if we are talking about a suicidal pilot, then we're talking about someone sadistic and deranged enough to kill 238 people. This is, by definition, not someone who is thinking in terms of what is the most rational response to a given situation, but a deeply fucked up individual. If they're sadistic enough to kill over 200 people and inflict untold suffering on their loved ones, maybe they're also sadistic enough to torment them for seven hours first. We're not talking about a simple suicide here, we're talking about a murder-suicide like Columbine or a suicide bomber.
Second, if someone wanted to steal a 777 for a terrorist attack, then why haven't we seen the attack? If there's a terrorist plot like during 9/11, then it requires the element of surprise. The more time passes, the more time there is for people to unravel the plot and send in the Special Forces to take out the plotters.
Third, if it's piracy, why haven't we seen the ransom? If you're a Somali pirate, then once you've got the ship safely in the harbor in Somalia, you send a message to the owners of the ship that you're open for negotiation. Also, if you want to do the piracy thing, you have to find somewhere sufficiently lawless that they won't arrest you when you land. Even a place like Kazakhstan isn't lawless enough; you don't just need a repressive dictatorship, you need a failed state level of anarchy, somewhere like Somalia or the border regions of Pakistan and Afghanistan. And as for selling the plane on the black market, you might be able to pinch and sell a few parts... but this is the most famous airplane in the world right now. there's no way in hell you could sell so much as a rivet if it had the serial number on it.
Pilot suicide is, unfortunately, the simplest explanation that fits the facts. We've seen it before and it would fit what happened here. For the sakes of the crew and families, I desperately hope that's wrong, but it's been more than a week- if these people were alive, we'd probably have heard by now.