Marlow sees the "symbolic" skulls lining Kertzs hut? What reflections do those skulls lead Marlow to make regarding the nature of Kurtz downfall in the wilderness?

Kurtz lies in a hut surrounded by heads on stakes. Marlow is not very shocked at the sight. He takes this as an indication that Kurtz lacks restraint in the gratification of his lusts, a condition for which the wilderness is culpable. Marlow assumes that Kurtz was hollow inside and needed something to fill that lack.