Pat Garrett and Billy the Kid... the stuff of more songs, books, and movies than a man can count. You can read voluminously about it elsewhere, so I'll focus on some stuff you maybe didn't know...
Like: things didn't go particularly well for Garrett after he killed the Kid. He did manage to collect a $500 reward that various citizens and merchants had put up for Billy's death or capture, but it was widely believed that it hadn't been a "fair fight," and he was criticized about it for the rest of his life. (His own position was basically, "If you knew Billy as well as I knew Billy, you wouldn't make a fair fight of it, either.") In 1882 he came out with a book called The Authentic Life of Billy the Kid, but it was poorly ghostwritten and didn't do well. He was defeated for re-election as Lincoln County Sheriff, ran the next year for Grant County, New Mexico Sheriff, and was defeated for that. In 1884 he lost a bid for a New Mexico State Senate seat.
It was downhill from there. Drinking, gambling, debts, and tax problems... On February 29, 1908, Pat Garrett was shot and killed by a man with whom he had been having a property dispute. (The man was grazing goats on his property.) The man got off. Said Garrett was reaching for a shotgun, and pleaded self defense.
Pat Garrett was too tall for a commercially available coffin, so one was special ordered from El Paso. He was 57 years old.
When the movie came out in 1973, critics firebombed Dylan for the score. Atrocious, they said, though I myself thought it was great, totally capturing the sad, lonely desolation of the whole Pat and Billy situation. Well, sometimes I'm ahead of my time, and sometimes I'm behind the power curve, but this time it looks like I was right. The consensus today is that it's an OK movie with a great soundtrack.