Rancher's law / Dusty Richards.

Enhanced descriptions from Syndetics:

In a hot Arizona river basin, three bodies sway from ropes: three men executed by their fellow ranchers. Hundreds of miles away, in the muddy streets of ort Smith, Arkansas, a deputy marshal hunts down a killer--the first of eight outlaws on his list. Now, the manhunter is about to be chosen for a mission to Arizona territory. Major Gerald Bowen wants Luther Haskill to go undercover and find out if the Arizona lynchings were a matter of justice, or cold-blooded murder.
Bowen is on a campaign to bring law to the frontier, and Haskill is the right agent to investigate the deaths. But in Arizona, Luther comes up against a wall of silence and a family guarding a deadly secret. And in a land where some people get second chances and some don't get any, Luther Haskill must make hi sway through the good, the bad, and the damned--in pursuit of a cold-blooded killer with a plan of his own...