I have never understood how that conspiracy theory could be believed. Hindley's death may have seemed premature by general standards, but even to the public it was clear that her health had been declining for years. In 1999, Brady and Hindley collapsed in the same week - causing the tabloids to write hysterical nonsense about a 'suicide pact' - and Hindley survived a cerebral aneurysm then as well as suspected strokes and a heart attack two weeks before she died. In those final years, the papers were full of reports of chest infections, blood clots, angina, osteoporosis, hypertension, etc, etc. Virtually all of Hindley's health complaints were caused or exacerbated by her heavy smoking and she had been warned to stop, to no avail. She was a walking timebomb.
I think there is also a religious angle. Hindley received the Last Rites (twice), and there is no way that a Catholic priest would offer that absolution to a person who was not dying. She was afraid of facing God's punishment, of her soul being cast into Hell, of being 'unsaved' - and I remember her friend, Father Michael Teader, saying that he had tried to comfort her by saying "God has no favourites" (Romans 2:11). She is gone.