Billy the Kid – better known to history as William H. Bonney, although his real name was actually Henry McCarty – was actually something of a non-entity in his lifetime. Although he claimed to have killed 21 people, one for each year he was alive, it’s likely he killed less than half that number.
He was betrayed and killed by Pat Garrett under circumstances which are still a matter of debate. In fact, Billy wasn’t even famous until a year after his death, when his killer Pat Garrett published a sensationalistic biography titled The Authentic Life of Billy, the Kid. This was what made Billy famous. He became a legend of the Old West, later fighting Dracula and traveling through time.
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Okay, so he didn’t really travel through time or fight Dracula. The movies where he did are not that much more historically inaccurate that Pat Garrett’s book (which was, admittedly, ghostwritten).