All six mentions of the tire iron come from Ennis.
Great point, Chrissi. (Isn't it great to be able to search the story?)
I'm reminded of one of my favorite IMDb discussions: Ennis's Maledictions with CaseyCornelius. Briefly, there are three:
1) "You probably deserve it" in response to Jack's statement that he feared getting shot by Lureen or "the husband" every time he slips off to see the ranchman's wife.
2) Most importantly, the Mexico response, "All them things I don't know could get you killed if I should come to know them".
3) "Why don't you" (quit me).
I would add 4) "I can't stand this anymore, Jack" at the end of the Lake Scene, just before the dozy embrace.
In my opinion, Ennis
psychologically kills Jack. Why? Because Jack has broken their pact that they're not "queer". Notice their last night together, they're talking about their "women", Ennis's Cassie and Jack's rancher's wife. But, Jack ventures the "truth" with "...sometimes I miss you so much I can hardly stand it." Awfully close to "I love you".
The closer Jack gets to "queer", the more distressed Ennis gets. Thus, Mexico is the last straw. If Jack is queer, so is Ennis, and he can't stand that.
Ennis is predetermined to believe queer = violent death, because of Earl (and his expectation his father would have come into the motel with the tire iron).
In the story, the sequence goes from: Lureen's call--->believing tire iron--->end of call--->didn't know if it were tire iron or real accident--->OMT mentions the other fella--->now he knew it was the tire iron.
So, as Chrissi points out, all the tire iron references come from Ennis. I think the murder is a
projection on his part.