Although it seems as if Tess kills herself, she was actually killed by William Saunders.
The narrator of the story is Tess’s sister Bee, who describes what happened to a lawyer. Through flashback, Bee tells her story as she is being held captive by the man who kidnapped and killed her sister.
Bess suspects almost everyone at some point in the novel, but when William Saunders comes to visit her and tells her things about Dr. Nichols, she suspects him. William is clearly trying to throw her off.
… I wondered if Dr. Nichols had played me. Had the dog and the scruffy clothes at our first meeting been carefully constructed to present a certain image, which I had unwittingly bought? (p. 229)
Dr. Saunders is an obstetrician, so he is not the first person you would think of to kidnap, torture, and murder two women. After all, he seems like a very nice man. He jokes with his patients and is very polite to Bee. However, hiding under that gentle exterior is a cold, calculating killer. Bee trusts him because he is the doctor who delivered her sister’s baby, and he is kind to her.
Bee is suspicious when she learns that Dr. Saunders wore a mask when giving the C.F. injections in her sister’s experimental procedure. Why would he wear a mask when injecting her and when delivering the baby?