They’re climbing up a mountain because Ben is injured, but he’s the only one who knows the way out. As they’re climbing, Sasha manages to single out his line on a tree and removes the safety harness (zero clue about climbing equipment). He plunges towards a rock shelf, and we – the audience – are watching from just below the rock shelf.
There’s a parallel between the way Sasha’s husband and Ben die: one is a terrible, terrible choice; and the other, an easy decision.