Why are the characters becoming more beast-like or bestial as the novel progresses in And Then There Were None?

The characters in And Then There Were None are being picked off one by one by a brutal murderer. What's more, no one has the faintest idea who's doing all the killing. It's a scary situation, and as the bodies start piling up on the island, people become desperate. There's a dangerous predator on the loose, and the assembled guests of the mysterious Mr. Owen are his prey. Under the circumstances, it's no wonder that they start behaving like animals; they have to in order to survive.
Toward the end of the book, Vera Claythorne becomes hysterical, screaming,

Don't you see? We're the Zoo. Last night we were hardly human anymore. We're the Zoo.

But actually, the guests are more like animals in a big game reserve being systematically slaughtered by hunters. Slowly but surely, they've been stripped of every last vestige of humanity by the murderer; they're forced to dig deep into their innermost primal resources to stay alive. Agatha Christie uses a good deal of animal imagery to describe her characters in the story, and the longer the story goes on, the more the characters start to exemplify their animalistic traits.