Jack's death is not ambiguous, it's explained in the movie that the murder scene was just Ennis' imagination.
It took more than 10 views for me to be 100% sure, paying attention to each small detail...
Ang Lee has hidden it very well...
Hmm...I don't think it was meant to be ambiguous but I do believe that sadly Jack was beaten to death.
I remember the first time I saw the movie, I found it confusing, and I honestly didn't understand that part. Ennis was talking on the phone with Lureen, and then suddenly it showed the images of Jack getting beaten with tire irons. I know now that it means that Ennis knew in his mind, he had that feeling that Jack was murdered. But I don't think it was just his imagination. I remember an interview with Ang Lee and the actors where he said that they actually filmed the scene that is supposed to be right before Jack was killed.
The people that kill him see him around town with Randall and then that's how they know he's gay and so they follow him. They also filmed it where Lureen says on the phone outright that she knew about Jack being gay. However they decided not to use those scenes. But I don't think it's so they could make it ambiguous. I think it's just another thing that this film does so well. A lot of what happened is not shown, and a technique the movie relies on is to show a little bit and then have the viewer put two and two together and figure out what happened. They don't have to tell us and show us everything for us to know what happened. From the hints we're given, we can figure them out. Both in the short story and the film, it's not what Lureen said to Ennis on the phone, it's
how she said that implies that Jack was murdered. Her voice is described as being cold, like she had found out something about her husband. Also, at the end, it's clear she's holding back tears.
That is why I believe that Lureen found about Jack being gay because of the fact that Jack was murdered. That is how she finally found out after all those years, even though I think there had been rumors going around about Jack for a long time, Lureen was too busy with her work to really pay attention to what was going on with Jack. It is therefore only after he dies that she learns the truth. And then Ennis calls her, and she figures out that Ennis was not just a fishing buddy, he was the man Jack had been in relationship with all those years. She finally understood why Jack took all those trips to Wyoming, why Ennis never came to visit him in Texas instead, and what Jack had been doing all those years. She must have felt completely embarrassed and betrayed that her entire marriage was a sham from day one. Alma accidentally found out about early on, but ironically Lureen, a much more knowledgeable woman, didn't realize. I think only part of the story she told Ennis was true...the part about Jack having a flat tire and trying to pump it back up...but the other part about how he died wasn't. I think what really happened is those guys in the cutscene came across Jack and offered to help him with his car trouble as a trick. Jack probably accepted a ride in their truck, and once they had him there, instead of driving to where they told Jack they would go, they went to a field or empty area instead and there they beat him to death. Unfortunately, although Ennis was sometimes bordering on paranoid, and he was so afraid that he let that fear ruin his life, his fears were not just his imagination.
Unfortunately things like that do happen, and it's a sad and painful reality in this world. Matthew Shepard, a young gay man, was beaten to death in Wyoming in the year 1998. Another young man was beaten and then deliberately run over by a car in Texas less than a year earlier...in 1997. That kid wasn't even gay, but his attackers targeted him because he was different from them and called him "faggot" just because he wasn't a "macho" jock like they were. There are some horribly bigoted people out there in this world. Sadly, Ennis's fears were all too real...although I do believe he and Jack would have been safe if only they had run away to a big city and lived together. There they would be much more anonymous and the whole town wouldn't know their personal life. I think Ennis realized that at the end. But living in a more rural, conservative society, it was a very real possibility that homophobic monsters would come beat them to death.