What event causes Casement to join a mission in the Congo in the novel, Valiant Gentlemen?

While working in the Congo, Casement goes hunting for an elephant in hopes of harvesting the ivory. When the elephant comes through the trees, Casement has an epiphany about the way that the ivory trade has affected the Congo. He then joins a mission because he knows they are trying to help the natives, rather than exploiting them as slave labor.