There have been previous threads on this, but a forum search turned up only one just now.
According to my reading on the Tom Dooley (really Dula) case, the situation really was this:
Tom returned to his home near Lenore, N.C. something of a hero after the Civil War. He was putting himself about with the local ladies when he apparently caught the clap from Laura Foster, then passed it along to Ann Melton. Of course the exact route can't really be known, but that's how it appeared. Ann got PO'd about it and killed Laura, then asked Tom to help her dispose of the body — which he did, but got caught at it somehow and was charged with the murder. Ann was also charged, but bragged in her cell that the was "much too pretty to be hanged."
Sheriff Grayson — can't remember his first name — was a little sweet on Ann and relied on her version of what happened to pin the murder on Dula (Dooley), even though there was a fair amount of physical evidence that he wasn't the killer. Ann's testimony succeeded in getting Dula hanged and her released. Grayson then married Ann and lived with her for a number of years. People in the area say that on her deathbed Ann told Grayson something — nobody knows what, but they all speculate it was a confession about the murder — which absolutely shattered him. A short time later he just quit his post and disappeared across the Smoky Mountains into Tennessee.
Hope that helps,