The Authentic Life of Billy, the Kid is a biography and supposed first-hand account written by Pat Garrett with the assistance of ghostwriter Marshall Ashmun "Ash" Upson. In that book, Garrett and Upson give the date of the Kid's birth as November 23th, 1859, and say it he was born in New York City.
The problem is that though this information has been accepted as the actual birth date and place for Billy the Kid, no one has any proof as to when or where he was born. That means there is no record of the month, the day or even the year. So then, if I'm right in that there is no record, then there are some of us who want to know where that date came from? And really, how did Garrett and Upson know where since they did not really know the Kid?
One writer has said that Marshall Ashmun "Ash" Upson made up the year and simply gave Billy the Kid the same birth date as his own which is November 23rd. I don't know if that's true, but it does appear pretty fishy since no one can find a record of birth anywhere for the Kid. And yes, Upson was born on November 23rd.
As for the year, from what I can see, it's all speculation as to whether or not he was born in 1859. As far as some are concerned, since some have found witnesses that say that the Kid was actually 17 during the Lincoln County War in early 1878, then that means the Kid may have been born in 1860 or 1861.
Fact is, his the date of his birth remains a complete mystery. Yes, the same as with whether he was born in New York or Indiana or what have you. No one can say because there is no record of his date or place of birth.
Now as for how many men did he kill?
To my knowledge, the myth that he killed 21 men is not true. Yes, as for the killings that we absolutely know are attributed to Billy the Kid, that number is 4. We know that he killed Frank Cahill, Joe Grant, James W. Bell and Bob Olinger.
We know that he and other gunmen were involved in the deaths of Billy Morton, Frank Baker, William McCloskey, William Brady, and George Hindman. But even though that's the case, we do not know if he was the shooter or if it were one of the others whose bullet or bullets may have killed those men.
The problem here is no different than the myths of Doc Holliday, Wyatt Earp, Wild Bill Hickok, King Fisher, Ben Thompson, Luke Short and many others. A great number of legends have killings attributed to them that they simply did not do. In many cases, someone else did the killing and they were credited for it. In other cases, they themselves built up their legends by coming up with tall tales about people they fought and won, and struggles they overcame.
It is like not having witnesses to John Wesley Hardin's claim that he drew down on Wild Bill Hickok and that Hickok was fine with that. That was Hardin's claim from inside prison, and it is completely fabricated. If it isn't than someone should produce a witness, but there isn't.
In the case of Billy the Kid, it is believed that Deputy James Carlyle was mistakenly shot and killed by his own posse who they thought was Billy the Kid. And yes, it was shown later that the posse blamed the killing on the Kid out of convenience instead of owing up to a mistaken and friendly-fire. Any evidence that points to the Kid has been shown to be circumstantial evidence.
Billy the Kid has been blamed for killing Buckshot Roberts, but it was Charlie Bowdre killed Roberts. The Kid was credited with killing Charlie Crawford, but his killer was Fernando Herrera who is said to have shot Crawford as he rode towards town to join Peppin's posse during the siege at McSween's house.
Billy the Kid gets credit for the killing of Robert Beckwith, but no one really knows who's bullet killed Beckwith. And as for killing Morris Bernsteinring, he was killed by Atanacio Martinez and not the Kid.
So while we can say that he participated in five shootings, it's a lot more accurate to credit Billy the Kid killing the four men that we absolutely know he killed. And that, well that's no where near 21.
And yes, that's just the way I see it.