Phillip Lombard and Vera Claythorne were the last people on the island as far as they knew. They were both suspicious of each other because who else could be the murderer and they both knew it wasn't themselves
Vera decided to pull a fast one on phillip and grab his revolver because if he was the murderer he would easily have the upper hand on her so she decided to take his revolver.
Hugo is that you?
Phillip had realized that Vera had stole the revolver and Vera took the shot and killed him .
Vera left Phillip's dead body and went to the house feeling safe and victorious thinking that she was the last person on the island.
As the time went on Vera began to go insane. She started to hallucinate thinking that Hugo was with her.
Vera's craziness eventually drove her to hanging herself with a noose that was hanging in her room. The chair was pushed from under her and then there were none.