The "grove of death" - What's the condition of the people? What is Marlow's reaction? Should it be anything else?

Marlow is inspecting the sorry dilapidated station. He comes across a grove of trees full of sick and starving black laborers. They have become ill and, unable to work, are allowed to wander off and die. It is a picture of human cruelty and misery that Marlow had not experienced before. I thought Marlow's reaction was sufficiently disturbed even morbidly confused enough to appear understandable,

"While I stood horror-struck, one of these creatures rose to his hands and knees, and went off on all-fours towards the river to drink. He lapped out of his hand, then sat up in the sunlight, crossing his shins in front of him, and after a time let his woolly head fall on his breastbone."