On March 9, 1878, the group was pursuing two men by the names of Frank Baker and William Morton, who were allegedly the men who had killed Tunstall. When the Regulators caught up with the two men; however, they had other ideas rather than to bring them in and planned to kill them. However, a minor Regulator, who was also thought to have been a Murphy/Dolan partisan, William McCloskey, objected to the killing. MacNab then shot and killed McCloskey while Baker and Morton fled. However; as the two were trying to escape, they were gunned down, allegedly by Billy the Kid.

Less than a month later, the Regulators were involved in a shootout with law enforcement in front of the Lincoln County Courthouse on April 1, 1878. MacNab, along with Billy the Kid and several others, fired shots upon Sheriff William Brady, and his deputies, George Hindman, Billy Matthews, John Long, and George Peppin. When the smoke cleared, Sheriff Brady and Deputy Hindman were dead.

Three days later, on April 3^rd, the Regulators were again involved in a gunfight at Blazer’s Mill, which resulted in the death Regulator leader, Richard Brewer. MacNab then assumed command of the Regulators, but would not hold the position for long.   

On April 29, 1878, when MacNab, along with Regulators, Ab Saunders and Frank Coe were riding south of Lincoln, they were ambushed by the Seven Rivers Warriors. In yet another gunfight, MacNab was killed, Saunders was wounded and Coe surrendered.

After MacNab’s death, Josiah "Doc” Scurlock then took the leadership role of the Regulators and vowed to avenge Frank’s death. Several weeks later, on May 15^th, they invaded the Seven Rivers Warriors camp and killed Manuel Segovia, who had been the shooter in MacNab’s killing.