A rogue state like North Korea would have the capability, but they don't really have the motive since they can acquire passenger airplanes by other means. (North Korea's national airline has 27 planes -- if they wanted to make a nuclear kamikaze plane, they'd just use one of those.)
The hijacking and stealthily flying it to North Korea are so complex it's beyond ridiculous. There are so many more details which could go wrong and a much bigger international backlash especially from China. And why would they fly back across Malaysia if they wanted to go to North Korea?
On the upside if they are holding a bunch of really smart guys hostage and giving them stuff to build things with then a real Iron Man suit is surely inevitable.
If the last contact of any sort had been the 'good night message', then we would never have known.
Imagine I wanted to kill a person, and I've decided to use a car to do so. I can't use my own car for obvious reasons, so any stolen car will do as long as it doesn't connect to me.