If the transponder was turned off manually, it is virtually a certainty it was hijacked by one or both pilots.
If the plane was pinging automatically for up to 4 hours, then the plane was being purposely taken to a specific location because it didn’t fly until it was out of fuel nor was it destroyed early in the flight.
While there is a possibility the passengers and flight attendants figured out something was wrong after several hours and fought back, it is also likely they were mostly asleep. Also with the cockpit door locked, the passengers had no way to intervene. Being dark out, they wouldn’t have any landmarks as to where they were. Perhaps they only realized something was wrong when they didn’t land in Beijing. The passengers may even be alive and held as human shields.
If it was purposely flown to a specific location, it was intended to be flown out at a later time from that location.
If it was to be flown again, there has to be a source of sufficient jet fuel at the location.
If the plane was hijacked, the US likely has narrowed down the location and now has resources in place assessing the situation.
What will terrorists do with an airplane? Assuming they safely landed the plane somewhere in Asia, who are they going to attack wth it?
Even if they loaded it up with fuel and explosives, even nukes, if they planned to attack the west, they would still have to fly it half way across the world under the radar.