I'm curious why you think of this as such a concrete fact. It is hard to understand that you would consider it your right to override such a core component of human autonomy (the choice to continue living) because they owe you/society something.
If someone's suffering is so great (be it physical *or* mental) that they have reached a point where they personally would trade human life for non-existence, who are you to say "Sorry, we own you. You cannot make that choice"?
I am sorry if my comment sounds judgemental because I am genuinely interested in your opinion about who would ever be qualified to make that decision and decide "No, this person is bound to us."