IMO:
For me anyway, I not sure of what happen to Jack. But this if anything is a "cautionary tale". It shows what hate does, it show us what fear can do.
If Jack's death was a murder, a "gay bashing", then Ennis was right along, he had a reason to fear, after all he seen such things already when he was nine, then hate exist, end of story, (we know that it does exist, we seen it or was recipients of it).
But what does that hate, that fear does to a human being, how does control his life, influence him in the way he lives his life, and how it destroyed his life.
Ennis was a perfect messenger, through him, we saw what seeing hate as a boy can do a man, and because of it the hate of one self can grow. Ennis distant himself from the one true love of his life,and the painful struggle of that love which was life giving, life sustaining, and hopeful, and stifled because of fear. And in the end the death of his lover would always haunt him, the spoon in the can of beans that turned into tire irons. The manner of Jack's death is something Ennis would always be struggling with that image of Jack's dying in his mind, what was truth, and what was his reality, and what we can learn from it. My 2 cent.