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.
This question isn't as easy to answer compared to many other novels currently out there. The book And Then There Were None, although one of my favorites, is very difficult to define the protagonists as well as the antagonists.
The portagonist in a novel is of course the focus of the story, and is usually pretty clearly defined. In this novel, it can be looked at as if there were ten "main" characters, or protagonists. Each individual that is invited to the island, has their own story, information, and background to them. That is what makes this novel so unique. These ten characters, could also actually be looked at as the antagonists. It is a very interesting and rare situation where characters could potentially be looked at as either sides.
They could be seen as antagonists because they each are looked at as either enemies or a detrement to the other characters. For instance, Dr. Armstrong was thought by many to be the murder. If he was thought of in this sense, then of course he would be an antagonist to the rest of the characters in the novel. It all depends on perspective and making the argument on who really is that person. Unfortunately, this novel doesn't provide enough for that certain exact protagonist/antagonist format. However, it does let you decide and determine how you feel about the whole book!