The narrator in this novel is a third person omniscient narrator, meaning that the narrator is not part of the story, not one of the characters, and has full knowledge of all the characters and what will happen to each of them.
Because the narrator is the author, the suspense is built deliberately so that the mystery deepens and confuses the reader. It is really hard to figure out who is killing these people when no one can get on or off the island.
The fact that the narrator is omniscient also makes everything that she tells us about each character very important and the reader looks for clues to who the murderer is and why these people were invited to the island.
"As the narrator notes, nothing can be hidden in this house, especially the guilt of all the guests who inhabit it. The manner of death Wargrave chooses for himself is also symbolic, and he uses it as a clue to the real identity of the murderer on the island. He arranges to shoot himself in his forehead, the first time as a trick and the second time for real."