Buckskin Frank Leslie pleaded guilty to killing his girlfriend Mollie Edwards in January 1890. It seemed to be an open and shut case; Leslie killed her (see photo of grave) in a fit of rage. It may have been more complicated.
Leslie originally pled not-guilty, saying he’d suffered severe head injuries about a decade before. Leslie argued he had brain damage that impaired his ability to make rational judgements. But an expert from California failed to get to Leslie’s Tombstone trial in time to support that claim. Leslie changed the plea and did the time.