Naracott returns to the house to find that the killer is Emily Brent, who is really the famous actress Gabrielle Steele. Steele wanted Wargrave killed because he sentenced her true love to death. She would torture him first by killing those around him first, all criminals who had gotten away with their crimes, and she explains how she killed each one. She had faked her own death earlier.
who was the killer in then there were none
Emily Brent