Many Peirsol families that trace their ancestry to the Pittsburgh area have a tradition that states that an ancestor was killed by Native Americans. The name of the ancestor and the date and circumstances of the incident differ but it seems very likely that such an event actually did happen.
Following are the accounts that I’ve found so far. At least three of them stem from the same basic source, Joel Perisol, who was born in Pennsylvania and was the victim’s grandson. While the details diverge quite a bit, it seems that Joel’s unadorned account would be the most reliable.

+ "Petter Peirsol was the father of Joel Peirsol, the subject of this sketch, and was born in Pennsylvania in the year 1780 … Joel Peirsol was born in Pennsylvania in 1804, … His grandfather Peirsol was killed by the Indians in the year 1780, within eleven miles of Pittsburg." – "The 1871 Atlas Map of Fulton County, Illinois," by Andreas, Lyter, and Co., Page 35.
Slain ancestor: father of Peter. Location: within eleven miles of Pittsburgh. Date of incident: 1780. Date and source of account: 1871, from information provided by Joel Peirsol, the victim’s grandson.

+ From a biographical item on John C. Peirsol of Monroe Township, Monroe County, Mo., in 1884: "Mr. Peirsol comes of an old and highly creditable family of the country, tracing his lineage back through a line of ancestors who have brought no reproach on the name he bears, but have always held worthy positions in the communities in which they lived. The family has been settled in this country for nearly 200 years. His father’s great-great-grandfather Peirsol was one of three brothers who came from England to America in 1683 and settled in Pennsylvania, whence the name has radiated into different States. Mr. Peirsol’s great-grandfather, Peter Peirsol, was killed at Ft. Duquesne in 1753, when under the command of Washington, at the time the English or Americans were driven from that fort by the French and Indians. Peter Peirsol, Jr., was born after his father’s death, and he became the father of Mr. Peirsol’s father, Joel Peirsol. Joel Peirsol was born in Berks county, Pennsylvania, and after he grew up came West to Wayne county, Ohio, where he married Miss Catherine Emery. In 1836 they came to Fulton county, Illinois, where both parents lived until their deaths. The father became a leading and wealthy farmer of that county, and John C. was born there May 16, 1846." – "History of Monroe County, [Missouri]" 1884, available on the Monroe County RootsWeb Internet site at www.RootsWeb.com/~momonroe/history/chapter19.htm.
Slain ancestor: Peter Peirsol, father of Peter Jr. Location: Fort Duquesne. Time 1753. Date and source of account: 1884, from information provided by John C. Peirsol, victim’s great-grandson.

+ From biographical note on Benjamin Peirsol – identified as the son of the Benjamin Pearsall of Hampshire County, Va. – J.E. Peirsol of Tulsa, Okla., wrote: "My Father told me that my Great-Grandfather as killed by the Indians at old Fort Duquesne, the present site of Pittsburgh, Pennsylvania." This would refer to the younger Benjamin. Clarence Pearsall, editor of "History and Genealogy of The Pearsall Family in England and America,"states "It is more than probable that this tradition was meant to refer to the great-great-grandfather of the writer. It is however, also within the range of possibility that it may have been his grandfather." Pearsall then describes an attack by Native Americans in western Pennsylvania in 1792 and states "it might very well be that during this period of Indian depredations Benjamin Peirsol may have lost his life at the hands of the Indians." However, the only Benjamin who actually appears western Pennsylvania records moved to Ohio after 1800 and lived until 1824. – "History and Genealogy of The Pearsall Family in England and America," edited by Clarence E. Pearsall, 1928, page 1459
Slain ancestor: J.E. Peirsol’s great-grandfather [identified by Clarence Pearsall as Benjamin Peirsol]. Location: at old Fort Duquesne. Time: Not stated [but identified by Clarence Pearsall at 1790s]. Date and source of account: 1917, from letter by J.E. Peirsol, victim’s great-grandfather.

+ "Benjamin Pearsall was among those who came from Hampshire County, Virginia, and settled in the country south of the forks of the Ohio but west of the Monongahela. He located on Chartiers Creek in what is now Washington County, Pennsylvania, near Fromans Fort. … Hence one day [during Lord Dunmore’s War in 1774] when Benjamin Pearsall was working in the field with his grandson, Jacob, a band of Indians came suddenly upon them from the cover of the underbrush and while Jacob hurried to the house for assistance his grandfather stood his ground against the red men. Although he killed several of them they were too many for him and before his sons could come to his relief he had bee killed and scalped. The family tradition is that his granddaughter, Anne, rode bareback with her hair streaming down her back to warn the neighbors of the Indian outbreak." – "History and Genealogy of The Pearsall Family in England and America," edited by Clarence E. Pearsall, 1928, page Page 1443.
Slain ancestor: Benjamin Pearsall. Location: On Chartiers Creek, near Fromans Fort. Time: 1774. Date and source of account: 1928, from accounts provided by unidentified descendants to Clarence Pearsall.

+ "SAMPSON PIERSOL and his brother John watched six Indians burne there father Benjaman Piersol at the stake. They was hid in a thicket after a surprise by the band of red skins. The had onley one gun betwin them and if they would of tried to rescue there father they would have all been killed or captured. This took place at home where neer Fort Pitt – now were Pittsburgh, Pa, now standes. Then after that Sampson Piersol killed every Indian he could as long as he lived." This account, which is signed by Mr. E.E. Nye, was "told by his [Sampson Piersol’s] grand sons and grand daughters Joseph Piersol and Susie Grimm and others that could remember there fathers and mothers telling of there pioneer line." – Peirsol family file in the Beaver County Genealogical Society at the Carnegie Library in Beaver Falls, Pa. Written after 1941 since it mentions alterations to Sampson’s gravesite in that year.
+ Slain ancestor: Benjamin. Location: Near Fort Pitt. Time. No stated. Date and source of account: after 1941, attributed to Joseph Piersol, victim’s great-grandson but not recorded for many decades.

+ Jacob Pearcil, estate administration in Washington County, Pa., April 17, 1783. Mentions fees paid "for schooling & clothing for one of the deceased[’s] children" and to "Patrick Dugan for Boarding a Boy of Said Dec’d." Administrator was John Pearcil. – Washington County, Pa., Accounts, File P, No. 5.
+ "In the Spring of 1783, a band of twenty-five Shawnees, probably in revenge of the destruction of their towns by General Cleark in the autumn of 1782, entered Washington County and committed murders." Attacks on settlers occurred on March 27 through early April. "Pennsylvania Archives" mentions no Peirsols but only a few of the victims are named. – "The Indian Wars of Pennsylvania," by C. Hale Sipe, pages 680 and 681.
+ Possible slain ancestor: Jacob. Location: Washington County, Pa. Time: April 1783. Date and source of account: contemporary with death, official records.