So we have a crewmember who is a murderer, committing suicide, or a terrorist cell showing a government and other cell members how to do. Interesting either way.
Absolutely, interesting either way. Conjecture on our part, but at least our theories are tempered with boundaries defined by our knowledge of the facts.
If he (or they, or whoever hijacked the airplane) stayed at 8k cabin pressure using 100 percent, the oxygen supply wouldn’t last long. But up at 30k (at or beyond the practical limit) the oxygen duration is much longer. At 25k the regulator would pressure-flow at 100 percent O2, and above that the partial pressure drops, and so does consumption, therefore the supply lasts longer, to a point. Whoever did it needed to keep a high cabin altitude anyway for a time to kill off everyone (or almost everyone) in the back. He could run one air conditioning pack with full hot selected in the cockpit and the outflow valve full open, to keep himself (relatively) warm without pressurizing the cabin.