I found the story of Ann Girty's death that I read before. It comes from "The Indian Wars of Pennsylvania by C. Hale Sipe, 1929, p. 320. I'm not sure where the author got the story, but it may have been from a county history.
"Thomas Girty was the best of the four brothers. He took no part in raids against the Americans, but served his Country loyally. For many years he made his home near Fort Pitt, and was living in Pittsburgh in May, 1782, at which time he joined with other inhabitants of the town in a petition to General William Irvine, asking that the General order the soldiers of Fort Pitt to discontinue their practice of “playing at long bullets” in the streets, and thus endangering the lives of the children of the petitioners. This petition was granted.
Some time prior to 1800, Thomas Girty took up a tract of four hundred acres of land, a few miles south of Prospect, Butler County. Some authorities say he lived here until his death, which, they say, occurred prior to 1803, while other authorities say he died in Pittsburgh, on November 3d, 1820. Whatever may be the fact as to the time of the death of Thomas Girty, a settler, named David Kerr, laid claim to the Girty land, and, one evening in 1803, came to the cabin when no one was there except Ann Girty, wife of Thomas, and fatally shot her. Kerr had come for the purpose of ejecting Mrs. Girty. During the argument, which took place between them, Mrs. Girty struck Kerr in the face with a clapboard with which she was raking the fire, whereupon he shot her in the breast with his pistol. She died of the wound several weeks later. Kerr was never brought to justice for his crime, on account of the stigma attaching to the Girty name, and, for the same reason, the body of poor Ann Girty was refused burial in the Mount Nebo Presbyterian cemetery near her home. She was laid to rest in the forest, where the author has often seen her grave. Yet, the Butler County settlers bore testimony to the fact that the family of Thomas Girty were good and peaceable neighbors. Thomas Girty Jr., lived on the Butler County plantation for some years after his mother's death. On December 26th, 1807, he sold all his interest in the farm to Thomas Ferree, for a consideration of one hundred dollars, the instrument being recorded in the office of the recorder of deeds in and for Butler County, in deed book A, page 558."