We can't blame Jack's fate on one person, really, but we can definitely blame society, or, maybe more accurately, everybody whose looks turn sour and ugly when they realize what Jack and Ennis feel for each other. They're not just fighting one solitary bigot. They're fighting all of them, and in that kind of game you either lose (like Jack) or don't play (like Ennis). Either way, we don't really have ourselves a personified antagonist. No Darth Vader of homophobia here, just several low-grade bigots (like Joe Aguirre and Jack's Dad) who make their disapproval clear, and instill enough fear in Ennis to make sure their romance could never get off the ground.