[smokingfrog]

Terrorists could have killed everyone onboard with some kind of poison gas. Not sure how they could sneak gas masks onto the plane for themselves without raising some red flags though. I suppose they could use the oxygen masks that the flight crew might normally use in the event of a depressurization?

[gunsequalfreedom]

Not sure how they could sneak gas masks onto the plane for themselves without raising some red flags though.

Ground crew could have made sure they were on board.

[null and void]

Epi pens with atropine.

[Jack Hydrazine]

You can just depressurize the aircraft if it at cruise altitude very easily. It was a redeye flight so most were asleep. If the pilots gradually depressurized those awake would have gone to sleep, too. Everyone would have died a peaceful death in their sleep.