The option of death means that a person’s existence, his very being, he himself, must for the first time be justified. As long as there are any costs to living (and there always are, in terms of personal sufferings and impositions on others), the option to die leads him and those near him to ask whether his remaining alive is worth those costs. We may, in fact, conclude that he should choose (or should have chosen) death. But even if he and we conclude that his existence easily passes the test, that he is a valuable fellow to have around, he has been degraded from a subject to an object, from someone totally accepted to something that can in principle be rejected.
Once the availability of death makes a justification for staying alive necessary, moreover, that justification may be inherently hard to come by. Once told to choose, many dependent persons may (perhaps spurred on by rising resentment in their caregivers) find it hard to deny that the good they are doing for themselves and others is no longer worth the cost and imposition.
Indeed, once the gates have been opened, once the option of death has been introduced, once the necessary taboo against killing is removed, not just a few but most or all of us may sometime be unable to justify our existence in human terms. Do we really think that no one could find a better use to which the costs of our upkeep could be put? Are we so important as to be provably indispensable? The world will probably get along pretty well without us. That is what happens, after all, when almost anyone dies.
-Richard Stith, via Ramesh Ponnuru.