In 1877, Billy moved to Lincoln (New Mexico), where he was employed as a cowboy in the ranch of John Tunstall, a rancher and English businessman and his partner Alexander McSween, a well-known lawyer.
In this context, the so-called War of Lincoln County occurs, where following conflicts between Tunstall and McSween with another local businessman named LG Murphy (who was a member of the Ring of Santa Fe, powerful group formed by politicians, judges , businessmen, cattle ranchers and military men who controlled the territory of New Mexico with mafia methods). Tunstall had allied with John Chisum, also an important rancher, to face the Ring. The Ring hired Sheriff William Brady who with his assistants killed Tunstall in an ambush. The murdered rancher had been considered a new father by Billy, who along with workers and friends obtained arrest warrants, granted by Judge Wilson de Santa Fe, against the murderers of Tunstall, forming a persecution group that was known to Los Regulators These orders were later annulled by the district judge who was a supporter of Murphy, leaving the group in an illegal situation.
Billy killed Sheriff Brady and his assistant on Lincoln's main street, fleeing to Texas.
