SPOILERS for The Housemaid Is Watching by Freida McFadden

 

This post contains spoilers, proceed with caution. 

Ada thought she was the one who killed Mr. Lowell because he was grooming Nico. But really, it was the maid, Martha. She was in the house stealing jewelry. She was afraid of what her abusive husband would do to her if he found out she had been stealing, so she slashed Mr. Lowell's throat.