Hawke: Ride With the Devil by Robert Vaughan

Mason Hawke has a dark past that he's trying to forget, but now he must choose between wealth and power or saving a town that's under the rule of a corrupt lawman.

Mason Hawke emerged from war a scarred man, a man unable to return to a life of power and privilege. His only way out is to start his life anew, concealing his past from those he encounters. But things don't always go according to plan, which Mason finds out when he stops in a town where he knows the local sheriff, a man who has the town under his thumb. All he wants to do is settle down and pretend the past never happened. But now Mason has to decide whether to live the easy life, forgetting his past, or to risk sacrificing himself, and help the town break free of the tyrannical lawman, at the risk of exposing something he'd wanted to keep buried.