The clash of the misfit with conventional society

One of the major themes is the clash of the misfit with conventional society. Since society is composed of a bunch of conformist misfits, the misfit must learn to conform if s/he wants to fit in and succeed. In other words, if you want to 'make it' in normal society it's necessary to hide your problematic past & flaws and put on a front of respectability. Dad Longworth has figured this out and he's not about to allow Rio to ruin it for him.

Although it was wrong for Dad to take the money and leave Rio to be captured, one can't help but empathize with him. After all, he showed his honorable character a little earlier when he went back to pick up Rio when the latter's horse was shot from underneath him. This explains why Rio makes sure Dad is the one who leaves to bring back fresh mounts -- he owed Dad his life. When Dad finds the fresh horses you see his mind working: If he goes back for Rio there was a high risk of capture or death. The safest and most promising route was the selfish one.

The good thing is that Dad wisely took advantage of this opportunity and made a good life for himself in Monterey. He was obviously sick of his criminal lifestyle and wanted a real life, a real job and real family. He was getting on in years and knew that this may well be his final opportunity, so he took it.

The problem is that one's past always has a way of haunting us or coming back to bite us in the hind end, which is another theme of the film; hence, Rio walks back into Dad's life 5 years later and his American dream is suddenly threatened. Notice Dad's rage when he whips/beats Rio to an inch of his life; he's doing everything in his power to kill the ugly phantoms of his past, which is why he tries to get Rio hung on false charges later in the story.