The Lincoln County War in New Mexico was never as cut and dried as it might seem. Numerous men switched sides at various times.
Billy the Kid was among them, cowboying (and rustling) for the Murphy-Dolan operation and its allies the Jessie Evans Gang. In fact, he was jailed in late 1877 for stealing some of Tunstall’s horses. There are various theories on just what happened, but the Kid soon decided to make nice with Tunstall, got out of the calaboose and went to work for the Englishman.