I beleive Jack died in a gay-bashing because it fit so well with the story's theme of homophobia. From Earl's death to Aguirre's discusted look to Jimbo's put down, they all are hostile. Jack could easily be shot by Aguirre or bash to death by Jimbo and his gang of clowns. Plus, the things we seem to fear the most often happen to the people we love the most. But like everyone else, including Ennis, we will never know for sure.