I think it's important that this situation remain ambiguous to the viewer to a certain extent. I also think it's important that we see this in Ennis's imagination. The ambiguity that runs throughout the film on many different topics is part of what keeps the film intriguing. But, I do think it's expected that we have a hunch, even a strong hunch, that Jack really was murdered. The story about the tire exploding is just too preposterous (in my opinion). It sounds like a cover story to me.
I don't think this idea that it was murder is ambiguous to Ennis. His fear of just this situation is so intense (from the time he was a child) that I'm sure the idea that Jack was killed was overwhelmingto him. The analogy with the sheep is very good to point out. Yes, in the course of the film we see only the sheep and the two gay men killed in a violent and bloody way by predators. And, we see all three of these instances of violence through Ennis (he finds the sheep and we only know of Earl's murder through Ennis's memory just like we see Jack's murder in his imagination).
Jack's death is his worst fear coming true (at least in his head). And his conviction that it was murder through bashing adds to his sense of guilt and regret in the end. All of the discretion that Ennis forced upon their relationship (insisting that they live apart, etc.) was, from Ennis's perspective, meant to protect them from just this kind of violence. And, not only did this fail to protect them (Jack from the violence and Ennis from his grief) the discretion and circumspection caused them to lose *all that time* that they could have spent enjoying being together. They could have had 20 years of happiness together. This is a compounded tragedy.
So, yes, Ennis's internalized fear of violence against gay people has deeply impacted his and Jack's life. So as usual, I agree with latjoreme's statement above-
>Also, to me the movie focuses less on the threat of society's actual intolerance, real though that is, than it does on the effect that intolerance has on an individual: Ennis<
ps. I've just made my move over from the imdb boards (over there I'm "amandazehnder") and this is my first post at BetterMost. Glad to be here!