The enigmatic part of the text is as follows:
The old man spoke angrily. “I can’t get no help out here. Jack used a say, ‘Ennis del Mar,’ he used a say, ‘I’m goin a bring him up here one a these days and we’ll lick this damn ranch into shape.’ He had some half-baked idea the two a you was goin a move 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.”
So now he knew it had been the tire iron.
(Proulx - Brokeback Mountain 1997)
It seems that, to Ennis, there was a connection between Jack's murder and his wanting a divorce from Lureen and planning a sweet life with Randall.
Differing things can be inferrred from this: the possibility of being murdered because of wanting a divorce (and Lureen finding out the true circumstances), or a foolhardy wish for a change of lifestyle that led to an incidental murder.
The text is vague enough to accomodate both interpretations, IMO. But to be sure, Randall and Jack's attempt to live a sweet life with him is associated by Ennis with the certainty of Jack being murdered.
