Betty Neumar, 79, was married five times since the 1950s and each one ended with the death of her husband.
But eventually she was arrested in 2008, accused of trying to hire someone to bump off her fourth husband Harold Gentry, after his brother Albert begged cops to reexamine the case.
He said of her death: "I'm numb. I wanted justice and we're not going to get it."
She was released on a $300,000 bond and was expected to go to trial soon, although it had been postponed a number of times in the past.
Investigators had also reopened cases on her other husbands, but they will now close with her death.
Sheriff Rick Burris said authorities are looking into her death, ominously adding: "We're going to make sure we examine the death certificate."
Her son-in-law Terry Sanders said: "She was tough country girl and fought through a lot of pain. I'm not sure where she's going to be buried. Peggy and (her sister) Kelli, are taking care of all that."