Joe, in regards to the lyrics, they seem spotless except for some weird choices at line breaks occasionally and that I've always assumed the woman's name was "Rose" and not Rosa, though I suppose it doesn't make much difference.

The only other comment I would make is that Robbins recorded two versions of the song: a short(er) version (4:23) and a long(er) version (4:40). The only difference is that the long version includes the 4-line stanza that starts with "Just for a moment I stood there in silence" while the other one does not. Not sure which one was on the album or why they chose to remove the lines of moral reflection.

To answer the original question, I think Wikipedia's surmisation here is a good one: "The singer kills the newcomer, then flees El Paso for fear of being hanged for murder or killed in revenge by his victim's friends. In the act of escaping, the singer commits the additional and potentially hanging offense of horse theft... further sealing his fate."

I think the friends coming to kill him is more likely that murder by law until the horse theft. It was, like others have said, the Old West, and he seems mostly "justified" in the Old West's sense of things.