Kid Blue.

Bickford Waner, an apparently naive young man from Fort Worth, arrives in the tiny Texas town of Dime Box and takes on a variety of menial jobs. He's befriended by Reese Ford and his wife Molly, but before long Molly has seduced Bickford. Only with the arrival of Bickford's former girlfriend Janet Conforto is it revealed that Bickford is actually the notorious train robber Kid Blue. Humiliated by a scandal arising from his affair with his friend's wife, Bickford gives up on going straight and plots a crime.