The morality of survival

I feel that a recurring theme in the narratives we keep coming across is the questionable morality of certain actions in survival situations.  In Poe's Pym, we run across cannibalism in chapter seven, where Pym and his companions are forced to draw lots to see who is eaten among them.  And yet, despite how awful an act this is, I found myself accepting of their actions because of their desperate circumstances.  I similarly found myself approving of the actions of Simon in Touching the Void when he chose to cut the rope to save himself, and in Into Thin Air when rescue climbers chose to leave behind climbers on the mountain who couldn't be saved.  My acceptance of such things really leaves me wondering what else could I rationalize doing if my life depended on it?  I think this is a question that anyone would ask put in the right set of circumstances.  But what really strikes me about Poe's narrative is that his account is fictional, but ultimately believable.  There have been instances where sailors have had to eat other humans to survive because they had no other options.  This is what makes me believe that what Pym goes through could actually happen, and why I am able to rationalize their cannibalism.  Yet, I think that if this never happened in real life, I wouldn't be able to accept their actions.  I think that we the reader are only able to accept such low moral choices in fiction if they are based in truth.  Thus, despite Poe's, let say interesting, narrative choices, I am able to accept that Pym and company had to resort to eating each other in order to survive.  In survival situations like the one the faced, it happens to be a valid option.