Madison was being abused by her husband, the house said it would give her the power to kill him. She went to the pool as the house said. She turned into a mermaid and murdered her husband, along with three other men.
Episode 7 - Mermaid Tails
Madison was chained up on the second floor when the guest found her, she said she needed her daughters doll to be freed. She said the pool was guarded by her sisters, and she wrote the names of the men she murdered in her diary.