Killer Silver (Jubal Cade Western #4) by Charles R. Pike

The killing trail winds on for Jubal Cade, trained doctor and deadly sharpshooter. The hunt for the scar-faced man who killed his wife takes him through Laredo, to the Mexican mountains where Roberto Blanco and his gang of marauders have their hideout. Word is that Blanco's chief henchman is El Cicatriz—'the scar-faced one'. A man who enjoys cold-blooded murder.

Can it be that the long quest for vengeance is nearly over for the fastest doctor in the West?