Hays' most active period of rangering in Texas against the Comanches would last only about five years, from 1841 until February of '46. After he returned from Mexico he left the service for "personal reasons". Maybe he had grown tired of combat after the bloodletting in Mexico. Like I said, its a real pity he never wrote a book.
Birdwatcher
Others have speculated along the same lines.
In Mike Cox's Ranger history, he touches on that, as do other authors here and there. The degree and volume of butchery committed by Texas Rangers in Mexico during that war are often alluded to, but I've read no real details. I suspect that a very young (which he was, as you've pointed out!) and educated man would realize after such a horrific experience that killing folks ain't all that glorious a job, and decided that with a whole life ahead of him he could move on to a new career.
