Depending upon how one looks at the novel, Agatha Christie's And Then There Were None does not have a true protagonist. Figuratively though, the idea of the protagonist could be looked at very differently. Since each of the characters in the novel are murderers themselves (and are in turn murdered), the protagonist of the novel could be defined as karma or fate. Since each are responsible for the taking of a life, Karma (personified to represent a being who vindicates the deceased) takes the lives of the different characters.
Some readers may identify Justice Wargrave as the protagonist. Since it is his plot for justice which brings the others to the island. he could be seen as the one responsible for insuring their deaths. That said, given he himself is dead, some may not identify him as the protagonist either.