That’s a really good question! What’s fascinating for that period of history is – can we assign modern labels to historic behaviour when the societal norms were notably different? Technically you could label many gunslingers as serial killers but their motivations are far removed from the serial killers of today. Taking Billy as a good example:
– his first kill was arguably self-defence in a bar fight.
– his second, third and fourth were motivated by revenge/vigilantism in retaliation for murdering Billy’s friend John Tunstall.
– the fifth and sixth were lawmen killed in a shootout. Can’t really argue legal self-defence here but it’s certainly natural for a gunslinger to shoot back when being fired upon.

I think any murders after that are contentious but a pattern is already clear. He appeared to have a specific motivation for each murder that is not far removed from how most ‘normal’ people might react in the same situation at that time in the US. Therefore I would not classify him as a serial killer per se, I doubt he was motivated by killing or he would have done a lot more of it.

Of course, being a complete layman in these matters my opinion could be complete BS but there it is

I don’t know enough about every particular killing William Bonney committed to make a call, but I know some were part of the "range war". What I do know of him I’ve never considered him a serial killer. I think if the FBI labled him anything it would be a "mass murderer", not a serial killer. Serial killer has different connotations.

Outlaws like Bonney were organized criminals motivated by material gain and revenge. Very different motivation to serial killers. Think of the mafia, all about money and the bad blood stems from there.

Perhaps so, but apply the literal definition of a serial killer ?