That would solve the problem. Or put them out with sleeping gas.
Sleeping gas, my hairy purple donkey.
The passengers not in on this would be cranky and rebellious after landing. My bet is that they landed at a preplanned destination, where the passengers were herded off the airplane by a few people with weapons and then into a nearby pit, dug by heavy equipment. Either shot or just buried in the pit. A convenient cave would be excellent for this purpose.
There only reason they lived that long is that the hijackers did not want to clean the blood out of the plane, and bodies are heavy and hard to carry out.
My bet is on the plane itself is the item they wanted. Otherwise we would have had ransom demands already.
Live passengers need to be fed, need water, and in general are a pain the the fanny. If you release any of them after a ransom, it is a good bet that someone on board has navigation skills and has measured your latitude well enough that any military can find you in a few hours. There is always a chance that this knowledge leaves with the ransomed and there will be an army at your door in a few days.