he scheduled route took the plane to Beijing so it would have had enough fuel to reach North Korea
The plane is shown turning north-east just before disappearing, towards North Korea: http://www.flightradar24.com/2014-03-07/17:20/12x/MAS370/2d81a27
North Koreans have been known to do this, for example: http://en.wikipedia.org/wiki/Korean_Air_Lines_YS-11_hijacking
A man claims he called his relative after the disappearance and the phone rang briefly. North Korea is one of the few places in the world with total government control over mobile networks, but it's close enough to China that in some parts of the country you can get Chinese mobile reception.
The North Korean government is bat shit crazy. There's no telling what crazy logic they might have for taking a plane. They literally have no grasp of reality.
If North Korea did want to hijack an airliner it would be best-off choosing flights farther away to avoid arousing suspicion. It was a long-haul flight. The perfect place to perform the hijack would be over the sea soon after takeoff (where it was last seen). This would ensure a clean getaway with minimal radar coverage.
North Korea has supposedly friendly relations with Malaysia including relaxed visa regulations. This might make it easier for North Koreans to freely plan and execute an operation from Malaysia. The operation might include buying stolen passports so they wouldn't show up as North Korean on the passenger manifest. If that was the case then the mission would be to hijack an airliner and bring it back to North Korea without anyone suspecting North Korea.
What would you do with an airliner? Previously they've used kidnapped civilians as bargaining tools. You could even feasibly imagine them putting a nuclear weapon on the plane and sending it on a suicide mission to another country. The aircraft would be picked up on radar and as it was completely unexpected the airforce of whatever nation would send jets to investigate. The jets would report back the tail-number of the plane and there would be shock as it was realised that the missing airliner had come back from the dead. It's not too hard to imagine that there wouldn't be time to react or think - they would try to make contact with the plane, either the Korean suicide pilots would respond, or pretend to be unconscious, or even duck and hide.