They can at any time reduce their consumption of oxygen by 20%
Yes, this will be the stuff of Philosophy 101 classes for generations to come.
“The Trolley Problem” is now passe.
“Consider, what if one is trapped in a submersible with four other people, with a finite amount of oxygen...”
“They can at any time reduce their consumption of oxygen by 20%«”
Actually, no. A dead body consumes more oxygen in a closed space (until putrefaction is complete) and also produces methane and other poison gasses.
If they could kill a passenger and throw the body out it would increase supply of oxygen - but a dead body inside that thing would kill everyone else faster.