Someone's going around bumping off the guests on Soldier Island. He or she is picking them off one by one. This generates an atmosphere of sheer terror on the island. Everyone is scared out of their wits; anyone could be the next to die. Under those circumstances, it's not surprising that the guests become more animal-like in their behavior. They have, after all, been turned into prey by the mysterious murderer. In this brutal fight for survival, the guests need to shed what makes them human and embrace their inner animal if they're to make it off the island in one piece.
Toward the end of the novel, the hysterical Vera—who's described as "like a bird that has dashed its head against glass and has been picked up by human hands"—screams
Don't you see? We're the Zoo. Last night we were hardly human anymore. We're the Zoo.
Actually, it'd probably be more accurate to describe Soldier Island as like a gigantic game reserve in which the guests are there to be slaughtered by a big game hunter. But Vera's main point is valid; she and the other guests have been systematically stripped of their humanity by the ruthless killer stalking the island.