I don't think it was a state level actor. I think the most likely explanation in the event of a hijacking is rogue elements within the ISI (which is pretty common -- there are lots of factions within Pakistan, almost none of whose first loyalty is to the nation of Pakistan) providing cover and funding to a terrorist group. None of this would be unheard of; elements within the ISI protected bin Laden for years.
Stealing an empty plane from an airport would likely get you shot down before you made it 100 miles; air traffic is heavily controlled at passenger airports. If you want to break free of the air traffic system, you do it in the air over a sparsely populated and/or mountainous region. Older planes are expensive, especially large ones in working order with the range and cargo capabilities of a 777 (a working 777 would cost more than $200 million and be accompanied by a lot of paperwork.)
All of this is speculation anyway; we don't even know if it was hijacked or if it even landed if it was. It's just fun to play armchair intelligence analyst in the absence of good information.
Why steal a plane if you could just buy it (assuming state level actors can gain control over a shell company with an air operators' certificate; aircraft cost really isn't an issue when for scrap value you can get old 747s with plenty of range and capacity) or charter it (leaving you with only pilots to dispose of, probably on the ground, if you're looking to do something nefarious)?
The first rule of being an armchair intelligence analyst is to rule out the ridiculously silly, and "maybe they just wanted a 777-200" falls into that category along with "maybe those 9-11 planes were just a cover for a controlled demolition"
Stealing an empty plane from an airport would likely get you shot down before you made it 100 miles; air traffic is heavily controlled at passenger airports. If you want to break free of the air traffic system, you do it in the air over a sparsely populated and/or mountainous region. Older planes are expensive, especially large ones in working order with the range and cargo capabilities of a 777 (a working 777 would cost more than $200 million and be accompanied by a lot of paperwork.)
All of this is speculation anyway; we don't even know if it was hijacked or if it even landed if it was. It's just fun to play armchair intelligence analyst in the absence of good information.