SPOILERS for The Housemaid's Secret by Freida McFadden

 

Warning! This post contains spoilers. Proceed with caution. 

Wendy had set Millie up. The person that Millie knew as Douglas was really Russell, the husband of the real Douglas's secretary.  It was her, Marybeth, that killed Wendy and Russell, but with some encouragement from Millie. 

Wendy had killed the real Douglas and set Millie up for his murder.