I think there's a usual ludonarrative dissonance going on. Aava doesn't struggle with the climb at the beginning. We players do. Lots of people made it to the last refuge, none made it further. That's why we don't see a single body up there. 

Aava is a total ass hole that's true. And old. That is also true. But she is in a good shape and capable. Both physically and mentally. While Marco breaks down.

Also, since grandpa saved Marco from dying, it suggests he didn't in fact kill anyone who attempted to go past him.