Just seen a documentary about Billy the Kid on BBC2 it was well done and really interesting. It added depth to the usual story, as we are finding out more and more with the "wild west" history is that it was a very corrupt political society with the history written by the winners. Billy was less a psychopath than victim of circumstance, who wasn't smart enough to escape his fate when he had it seems several opportunity to do so. The story has strong links with Ned Kelly particularly in Peter Carey's telling, of very young people (kids really) lost in a vast continent at the whim of corrupt local law enforcers either constables or sheriffs.
For more details see this site, his story is still shrouded in mystery.
One last point I only noticed from the film is that in the famous picture of him (above) he is wearing a cardy. It's an item of clothing missed out of most westerns, probably because they were shot in Spain or Arizona and Clint et al would've fainted in the heat!