I can't imagine anyone seeing Brokeback Mountain as a story of a man who was a crazy paranoid. I believe Jack died by accident, as I said in that debate at IMDb. But I see BBM as being, in part, a story about the damaging effects of rural homophobia, in the oft-repeated words of another participant in that IMDb debate. (It's not just about homophobia; it's also a love story.)
But to the extent it is about homophobia, it is about how that societal prejudice corroded a man's life and mind and soul to the point that he couldn't be with the one he loved and ruined both their lives. The homophobia was very real -- Ennis encountered, at a very young age, about as brutal example of it as you can imagine. And Ennis' reaction to that experience, as well as to the experience of growing up with a dad who was a possibly murderous homophobe, was not at all irrational. His reaction makes perfect sense under the circumstances.
But it forever damaged the way he saw life. And in the end, it also affected the way he saw death.
To stress: People who argue that Jack died by accident are not saying homophobia doesn't exist, in life or in BBM. They're saying that this movie shows how homophobia leads to violence, both physical and emotional.