Why did Longworth double-cross Rio?

Going back to rescue Rio would be much more risky than just disappearing, keeping all the money would be an inducement as well. He is showing his true colors in keeping with Rio's later character analysis of him as a 'one eyed jack'.

I agree. When Rio's horse was shot down, Dad risked his life in a hail of bullets to come back to pick him up and carry him to temporary safety over the crest of the hill. Dad did not have to do that. And this explains why Rio honorably allowed Dad to leave for fresh horses -- he owed him his life.

Moreover, although it was wrong for Dad to take the money and leave Rio to be captured, one can't help but empathize with him. If he went 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; 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.