Or they were hijacking a plane to stuff it full of chemical weapons / radioactive material (think dirty bomb) with the intent of flying it over a major city and detonating it. It would be hard to do it in a western nation, but if you were aiming at a city near the "frontier" of India, Pakistan, etc. a bad actor could likely execute the plan before the jets they scramble could intercept the jet. There's a lot of bad stuff you could do with a flying metal tube capable of carrying 200,000 lbs of whatever you can get your hands on.

I don't think it was a state actor though. If it was a state actor, who would it have been? My bets are on Pakistan if it was a state actor; they're one of the only ones with a sophisticated enough intelligence organization to be able to hide a plane like this. In many cases, it's hard to tell where the ISI ends and the Taliban begins. Though the same could be said for Iran and Hezbollah, though the Iranian government has taken a reconciliatory tone with the west as of late and I don't think they'd risk it. It likely wasn't North Korea; the social engineering required would be too high and the North Koreans are largely incompetent at covert operations outside their borders due to their xenophobia. I don't think many other states with the capability to do something like this would risk it; the amount of publicity on this is too high.

If the plane was hijacked and successfully landed somewhere (and that's a big if -- IMO it's just as likely that it's at the bottom of the ocean and will never be found) it was probably a coordinated effort between a terrorist group and rogue elements within the Pakistani ISI. I don't think the older pilot was involved; there's been a lot made of his links to a controversial opposition leader in Malaysia, but his ideology doesn't align with any of the terror groups (Anwar Ibrahim is considered a liberal reformer) and all of the "controversy" around him is corruption-related -- not the type of thing that would cause someone to shift toward radical Islam.