I just wrote this over at the IMDb board. Silly me, still trying to slay the trolls, I know, but the good guys got to discussing this very topic and of course I *had* to jump in. Here's my $0.02, and then some:
While I love ambiguity in movies for the reason Diana and Jake touched on at the Aero screening (I'm not one of those people who has to have a reason for everything - I think life is random and there often is no reason why horrible things happen, but I agree that's hard for a lot of people to take), it's quite clear to me that Jack was murdered. I really believe that the scene in the bar early on where Jack tries to pick up Jimbo the rodeo clown, then Jimbo goes and says something to his buddies at the pool table about him and they all turn to look at him ominously is intended to be foreshadowing of that murder. That was not in the short story. And the first time I saw the movie, I thought right then, "Oh, no. Jack's gonna get himself into trouble later, isn't he?" When I saw the lamb slaughtered by coyotes later on after they first consummate their passion, I knew it for sure. That also was not in the short story.
In the short story, Ennis imagines Jack's murder while talking with Lureen on the phone, and then later, when he's at the Twist house in Lightning Flat, as soon as John Twist says that bit about "...and then this spring, he's gonna bring some other fella up here, some rancher neighbor a his," the narrative says, "So Ennis knew it was the tire iron."
I also think that if Jack really just died accidentally, the movie is not on the level of a Greek or Shakespearean tragedy like I'm convinced the screenwriters and Ang Lee want it to be. The real tragedy is that the very thing Ennis most feared would happen if they were open about their love happened anyway, or maybe even BECAUSE he was too afraid to be open about it. That's Romeo and Juliet. If it's just an accident, it's Love Story or Brian's Song or (gag) Titanic. And Romeo and Juliet was what they were going for. I absolutely believe that.