Death on the Nile - Agatha Christie

Before the murder happened in the first half of the story, the author spent a lot of time on the character and emotional flow of the three protagonists, which is different from the tough guy style of detective novels and belongs to the delicate female mind. The ending is unexpected, and the killer's ending is tragic. Polo sees the end but fails to prevent it. What is the duty of a detective? Bringing the killer to justice or not letting a life end?