Sam is still within the first area of the forest.
Even though he killed a bunch of people here, they didn't cover the whole first part of it. They covered a large part of it, but there is still some area left for him to explore.
And he went to the right. Even though he didn't know what is awaiting him there, occupying that area will make it easier for the base camp or whatever he is building.
Only after he went there did he understand that it might not be one of his smartest decisions. Because there are a bunch of monkeys awaiting his arrival.
As soon as he appeared within their zone, they jumped off of their hiding places within the trees and started attacking him.
Their teeth and claws became sharper and harder to the point that one of the monkeys's caught the reaper's sword with its mouth and he was slashing at that time.
