Read the book before you watch it
A lot of these reviews on IMDB say how it wasn't explained how the murderer did it, and thought it was quite confusing. They also couldn't believe that it was an Agatha Christie, from watching the film.
The film added a few parts, such as nudity for example, which would naturally make people think this.
Well, the book explains it more - it explains why he killed the victim, what happened to Miss Townsend, etc. Because it is from Mike's perspective, you understand more of his feelings towards both Ellie and Greta.
Also quite a lot of important dialogue is missing in the film. Apart from Ellie singing "Endless night" the film didn't really make clear why that song is relevant.
I think the film assumes that you already know what happens, what Mike is thinking etc.