I think Ennis wanted to beleive that Jack died the way Lureen tells him. I think he honestly thinks it was the tire iron. I think my mind has changed alot since reading Ennis and Ellery. I think that Ennis knew Jack was murdered but did not want to accept the fact he was killed because he loved another man. Sometimes it is easier to just be in the "open space". It is easier to deal with than reality.
"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. " In other words, there were things Ennis tried to believe but could never verify to a certainty because there no way to do so.
It's not that Ennis tried to believe things that were unrealistic, but that there was no way for him to be certain because he simply didn't have sufficient information to verify those things to a certainty, one way or another.
Jack father tells Ennis, "this spring [Jack’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 Ennis now knew that Jack had been seeing another man. Curiously, Ennis's (primary) conclusion from this revelation from Jack's father was not that Jack had quit him for another man, but that Jack had been "ranching up" with another man and murdered because of it: We read, "So now he knew it had been the tire iron. " Of course, Ennis doesn't actually know any such thing. Ennis doesn't know any more about the circumstances of Jack's death than the reader. But Ennis had been set up since childhood to believe that the "tire iron" was the probable fate for guys that "ranched together."
The written story leaves little room for the reader to doubt that Jack's death was a real accident: We read, "Ennis didn’t know about the accident for months until his postcard to Jack saying that November still looked like the first chance came back stamped DECEASED.” It was only Ennis that didn't know for sure: "He didn’t know which way it was, the tire iron or a real accident, blood choking down Jack’s throat and nobody to turn him over. Under the wind drone he heard steel slamming off bone, the hollow chatter of a settling tire rim." Note that the story describes Ennis imagining a scene of Jack being struck with tire rim, not with a tire iron.
But the movie assaults the viewer with a heavy-handed image of Jack being struck with a tire iron. It's no wonder many viewers walk away with the conclusion that the image of Jack being struck with a tire iron was a news flash rather than an image, as Ang Lee explained, "of what Ennis was feeling at the moment." In an interview, Larry McMurtry acknowledged that Jack's death could have happened the way Lureen described it, and Diana Ossana admits that they tell people they simply don't know how Jack died. Annie Proulx said the movie's version of how Jack died, whether by tire iron or tire rim, was ambiguous to her.
