That's a very interesting post latjoreme. Thanks for the quote from Ellemeno.
I do think that Ennis probably carries a lynch mob around in his head, but I disagree with the idea that he alligns himself with them. I think it's the opposite. I think he monitors his behavior, checks himself when gazing at Jack, and especially censors his speech (or performaitively says "I ain't no queer") out of fear that the lynch mob is constantly after him. In the book, while he and Jack are in the motel he says that if his dad was alive and saw them in bed together his could clearly imagine his dad taking that tire iron to them both. I think that dead and mutilated sheep set off all sorts of fears in Ennis's mind that he too is now one of the potential "sacrificial lambs". It's important that this vision of death by predator loss comes the morning after Ennis lost his virginity.
His paranoia seems to be come more and more obvious as the film goes on. This is clear in the conversation with Jack where he says he worries that people on the pavement look at him "like they know." He doesn't say what they know. But here, I think Ennis is worried that people can tell he's gay by looking at him somehow. By this point, whether he likes it or not, I think he's truly beginning to think of himself as gay. Also, during the argument scene his threat that "all those things that I don't know might get you killed if I come to know them" is interesting in how he phrases it. He already knows that there are things that Jack is hiding from him. He already suspects that Jack probably sleeps with other men to get by between their camping trips. He just can't handle actually hearing Jack admit it. Ennis likes to control the limits and character of the state of denial that he lives in. But, these two moments - the "people on the pavement" moment and Ennis admitting he already sort of knows the things Jack thinks he doesn't know- show that somewhere deep down Ennis knows the difference between denial and some of the basic aspects of their relationship.
I still think it's particularly disturbing that he'd choose to talk about killing Jack given his suspicions about his father. But, I absolutely don't believe that Ennis would ever hurt Jack. No way. The fact that Jack also doesn't flinch, even slightly, also shows that this doesn't really scare him either. In fact, he pushes Ennis further with the discussion after this *empty* threat.