And Then There Were None is, in many respects, a psychological thriller, examining themes of guilt and culpability through its various characters. Each of them is responsible for the deaths of other people, but each has evaded legal punishment for their crimes (and it is for this common ground that the murderer targets them for death). They are trapped together on an island, and they know that the murderer is one of their own. What results is an atmosphere of fear and paranoia, as no one on the island is in a position to trust anyone else, not knowing which of them is the murderer (save, of course, for the murderer himself).

One of the book's central conceits is its use of an old song, by which the murderer bases his various kills on the various deaths of the characters in the song (it is from this song that the book has its various names). In the rhyme, the characters in the song go into a zoo and one gets hugged by a bear, leaving only two survivors left. Musing on this particular part of the song, Blore notes that the killer will find it difficult carrying out this murder, given that there is no zoo on the island. Vera's response is that they themselves are the zoo.

I'd suggest that this is actually a critical line within the novel (reflecting some of its key themes about guilt, paranoia, and human psychology). As Vera notes, the killer does not need to find a zoo: the characters themselves are more than qualified to adopt that role for themselves, regardless of whatever pretensions to the contrary they might claim.