Yes, if one only watches the movie, there can be some doubt about how Jack died, but what came first, the Story or the Movie? The movie is based nearly 90% on the actual short story and the story leaves little doubt. Ennis believes Jack was murdered, but he doesn't really know and no one ever will except the people who found Jack after the accident or murdered him, if he was indeed murdered.
From the story:
Jack's Daddy:
"....He had some half-baked idea the two a you was goin a come up here, build a log cabin and help me run this ranch and bring it up. Then, this spring he's got another one's goin a come up here with him and build a place and help run the ranch, some ranch neighbor a his from down in Texas. He's goin a split up with his wife and come back here. So he says. But like most a Jack's ideas it never come to pass."
Narative:
"So now he (Ennis) knew it had been the tire iron."
Narative: End of the story:
"There was some open space between what he knew and what he tried to believe, but nothing could be done about it, and if you can't fix it you've got to stand it."
Comment: From the book, it's clear to me at least, that Ennis doesn't know how Jack died, but believes it was "the tired iron" / murder, but like I say, no one but the people who found Jack dead from an accident or found him murdered or the murderers themselves, if he was murdered, will ever know.
I believe Lureen. It was a sad accident. I really hate the gnawing ache the rises in my stomach when I think he was brutally murdered. It's sad enough that he died so suddenly leaving Ennis without him, sad enough for me. I believe Lureen. But belief, being what it is...often of the imagination, often not based on facts or reality... is a personal choice.
I respect Ennis' choice to believe what he believes. I respect anyone else's choice too. That's part of the beauty of fiction, isn't it...
Rayn