An account in the Pennsylvania Gazette of 13 May, 1757 notes that a William Walker was killed by Indians at McCormicks Fort on the Conodoguinet.
May 13, 1757, William Walker and another man were killed near McCormick's fort, at Conodoguinet
From this we can observe that
The above suggests that the person we are looking for was an adult in 1757 and probably lived near "McCormicks Fort", on the Conodoguinet. A "McCormicks Fort" is known to have been constructed in 1778 in the Juanita River Basin  but its location in the Juanita River watershed, as well as the fact that it was constructed 20 years after the event discussed here seems to preclude it from being the "McCormicks Fort of Interest. Direct evidence for where McCormicks Fort was located "on the Conodoguinet is needed, but has not been found. We can, however, make some guesses as to its location. First, it was presumably located on the land of someone named "McCormick". There are several McCormicks in Cumberland County as it was defined in 1757, as revealed by land warrants See: Analysis:Location of McCormicks Fort on the Conodoguinet. Some of these warrants, such as that for William and Thomas McCormick, lay in what is now Juanita County, and can be excluded from consideration. Others' such as a warrant issued to Alexander McCormick, are too late (1762) to have been the site of McCormicks Fort.
The best candidates for the settler on whose land McCormicks Fort was built are James McCormick on the Upper Conodoguinet (Franklin County), and John and Samuel McCormick on the Lower Conodoguinet. Currently, interest is focused on the the latter possibility.
It seems likely that the William Walker who was killed was one of the Letterkenney Walkers who settled in this area at an early date. A number of Walkers are known to have come to the area after about 1734, and most, if not all, are believed to be numbered among the Letterkenney Walkers.
Among the Letterkenney Walkers there are a number of candidates for this particular William. These include:
There are several other William Walkers of the right age among the Letterkenney Walkers who settled on the East side of the Susquehanna, but it seems unlikely that they would be on the Conodoguinet in 1757, and are not currently considered candidates.
Our "short list" of candidates is given in the sections below.
William 186 was the son of James, and believed to be the husband of Elizabeth Curry. His DOD is sometimes given as 1757, and sometimes as 1768. The former is presumably based on the belief that he is the William Walker killed by Indians at McCormicks Fort. The later is probably based on the statement in Source:Egle's Notes and Queries which gives DOD as prior to 1768.
The William Walker killed at McCormicks Fort is probably NOT William, son of William 186. Born in 1721 he would certainly be of an age where he could have been in a position to be the William KBI'd. This seems unlikely, based on the DOB's in the child list of William 186, as given on Ancestry Family Tree's as of June 2010.
A summary of that child list includes the following children of William Walker (186) and wife Elizabeth Hoge
Since known children of William, son of William 186 were born well after 1757, it seems unlikely that he could have been killed in that year. The above DOB's, however, have not been verified, and additional work is needed to rule out the possibility that William 186 was killed in 1757.
William 202was born about 1721, the son of William 186 and married Elizabeth Hoge.
William 200, b. c 1716, was the son of person:Robert Walker (52). Robert was the brother of William 186. Given his DOB of 1716, we would expect him to have married no earlier than 1746, and might have had as many as five children by his death, if he died in 1757. Our current child list, as shown on Ancstry, includes John, Thomas, William and---
This William would have been an married adult in 1757. Given his DOB of 1716, we would expect him to have married no earlier than 1736. We have a short child list for him, including John, Thomas, and William. John's DOB is commonly given as 1748/1755, and William's as 1750. These DOB's are consistent with him having been killed in 1757. The fact that the child list is short, is also consistent with someone married after 1746.
William ZZZ is said to have been born about 1747, and was the son of William 202, son of William 186.
William (YYY) was born about 1739, the son of James son of Robert son of William 186.