Rustlers on Horseback(1950)

When he catches wind that bookish George Parradine (John Eldredge) is actually a ruthless outlaw who's had one man killed and is now trying to steal a fortune from another, U.S. Marshal Rocky Lane (Allan Lane) poses as a bandit and infiltrates Parradine's gang. But Rocky's quest for justice is jeopardized when the dead man's son (George Nader) also goes undercover to get revenge on his father's killer. Fred C. Brannon directs this 1950 Western.