Read this: Butch Cassidy, My Brother

Lula Parker Betenson attempts to set the record straight about her brother, Robert, who went on to fame/infamy as Butch Cassidy.It's a short book and a good read, with a lot of details and anecdotes about western life in the last 19th century.
Betenson does a good job of trying to explain how her brother ended up on the wrong side of the law. She is protective of her brother while making few excuses.
As to Cassidy's demise, Betenson says he died 17 years after the Southern American shootout which supposedly took his life.