Hmm... actually, an interesting preliminary question (I think this could be refined to better show what xeno is trying to illustrate). But this isn't an unrealistic question. Actually, there was a very near example of this in the Nazi concentration camps, which is why I suspect xeno used the Nazis in their example. (Yay! I actually took a course and wrote a paper about this particular subject... never knew it would come to use!)
There were at least some people who were placed in this type of scenario: the Sonterkommando in Auschwitz and other similar Nazi camps. Essentially, a group of Jewish prisoners were selected to facilitate the mass execution process... for example, by cleaning a gas chamber after an execution or removing gold teeth and other valuables from bodies. In effect, these prisoners would actually take part in the extermination of their kin, whether they liked it or not.
These prisoners were in no way guaranteed longevity in exchange for their service. At Auschwitz, Sonterkommando units were generally replaced every 3-4 months, with the first task of the incoming unit to be the execution of their predecessors. Considering that normal prisoners at Auschwitz were not deliberately chosen to be executed in a fashion similar to the Sonterkommando executions, if anything, this position actually reduced the prisoner lifespan.
What did the Sonterkommando receive for their work? Material comfort. They were generally given more food and amenities than the general population at Auschwitz. Among the prisoner population, these people were considered the wealthy high class of society.
That being said, remember what a lack of "comfort" means in this context. This wasn't a question of living in a studio vs. one-bedroom apartment. Being outside the Sonterkommando or another similarly privileged class in Auschwitz meant months of slow starvation and unending manual labor, with even suicide denied as a means of escape through various tactics (even the electric fences were made to be high enough voltage to incapacitate, yet low enough voltage to prevent death, of anyone trapped). So when I say "comfort," I mean "lack of perpetual torture, ended only when your captors chose to end it."
Correct me if I'm wrong, but perhaps a good way to make this question understandable is to put it in the historical context on which it actually did exist in real life. Given the choice (and I am not exactly sure that choice existed for most), would you be willing to spend your days as a Sonterkommando, as opposed to a general member of the Jewish prison population at a death camp which existed at the time?
(Disclaimer: I don't in any way mean to degrade the people who worked in the Sonterkommando units. In fact, the last Sonterkommando unit in Auschwitz actually organized the last major prisoner uprising against the Nazis in Auschwitz, although it was terribly unsuccessful for a number of reasons).
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