There's also some bit of (ironically) survivor bias going on here: Most of the people sitting in front of the ditch didn't leave the country. They didn't join up with the resistance. They didn't try to escape 3 weeks prior. This isn't even getting into the Jungian "death drive" where, at a certain point, you'd rather just die than keep trying to struggle.
Some people make bad, illogical choices when the stakes are extremely low, and some do the same when it's life or death. The human mind is a wonder of inefficient perfection.
> This isn't even getting into the Jungian "death drive"
The death drive is Freudian, not Jungian. However, it might be very relevant here. Freud's conception of it would include passively accepting death. As you say, by the time the prisoner is about to be executed, they're often resigned to their fate, the death drive having overcome the id long ago. Contrast with massacres where the victims weren't expecting it (the Malmedy massacre of freshly captured troops comes to mind from the same era) and you see people trying to run away once it starts.
Lacan called himself Freudian, but I think his conception links the death drive with active desire, seeking a form of self-destructive pleasure by action. I recall from Écrits that he links it to masochism, for example, calling it an expression of transgressive jouissance.
Some people make bad, illogical choices when the stakes are extremely low, and some do the same when it's life or death. The human mind is a wonder of inefficient perfection.