Terry's death is never fully explained in the film. The only time the audience sees Terry alive, during the laundry room scene, she is wearing Mrs. Castevet's antique pomander ball charm, filled with Tannis root. We see the same charm around Terry's neck in the pool of blood by her dead body. The audience is given no reason to doubt Terry's claim that she is a drug addict apparently "saved" by the charity of the Castevets. So, the presence of the charm, which Mrs. Castevet gives to Rosemary shortly after Terry's suicide, logically signifies that Terry, like Rosemary, was being groomed for a Satanic purpose of which she was initially unaware.