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Crazy. But the thing is... I can see how it could happen. How things could degenerate like that.

One thing of particular note is that it's never explained how this particular group of people came to be travelling together and came to be the only ones to survive, where they were coming from of where they were going to. Or even when it happens or how far they likely are from anyplace. This makes it a little strange at first until the premise sinks in as accepted without need of explanation, but it does a lot to emphasize the fact that from the the event just before the beginning of the story onward, any common history or knowledge that the characters might have had beforehand vanishes into unimportance. No past exist for them and they know only each other. The dynamics of what happens to them subsequently depend only on each of their natures and their interactions.