William Kidd is an old and honorable name. A famous one was Captain Kidd, privateer for the Queen. There is a professor at Cambridge, internationally known for his work on the tectonics and structure of the earth.

Billy the Kid generally used the name William Bonney, but it seems to have been something like Cutler originally- too lazy to look it up right now- or see the book by Utley.
The Lincoln County War, by W. A.Keleher, Univ. New Mexico Press, is the most authoritative book, with the legal background to the War which was involved and acrimonious, and excerpts from the pertinent legal and court documents. Unfortunately this book, prized by lawyers and New Mexico buffs alike, is very hard to get and therefore in the $200 dollar range.
Billy seems to have been a semiliterate saddle bum. He worked for one of the contenders; the entrepreneurs involved (one English, killed by a hired gun and saddle bum on the other side) would not stop short of murder and thievery. Would Billie's life have been that of a normal cowhand if he had not been involved in the War? Who knows.