This is a huge burden to be aware that you may be the reason for your beloved person to die in such a horrible way. I am here with Ennis 100% even he WAS guilty in a way, but had no power to turn himself into the person, only Jack death could create. But, his sorrow is so palpable, so deep and heavy, that no one can remain insensitive to that.
I liked Ennis in LF, even it is so anxious scene, that I rarely watch it. But he was the man we wanted him to be! Polite, but proud, sensitive, but firm... Love him there, even my heart is broken! And will always be each time I remember him in Jack's room...
I don't think Ennis blamed himself, nor is he to be blamed for Jack's death (whether by accident or murder). My impression of Ennis as portrayed by Heath is someone who was grieving for the loss of someone he loved, for the opportunity to love and be loved, because by the time he sent the postcard to Jack, he dumped Cassie and was ready to face this thing with no reins, this thing he finally had a name, love. It's why the screenwriters injected the diner scene. It makes the scene in Lightning Flats more poignant.
In a sense, the Ennis that went to Lightning Flats was the Ennis that Jack met, the reticent, quiet, withdrawn, polite, dutiful, follow the rules Ennis that would not stir trouble. The trauma of Jack's death, not unlike the trauma of losing his parents at an early age, put him back to familiar territory, the withdrawn cowboy. That's why he suffered in silence when the old man bullied him with the Randall bits. He was angry, he was hurt, but he did nothing, and suffered in silence as he always did...gotta stand it if you can't fix it. But fortunately, Jack's Ma stepped in, as she must have done often with Jack, placed a gentle compassionate hand on his shoulder, and guided him away from the bully to find refuge upstairs, in Jack's room. She kept it like it was when he was a boy...a refuge away from the angry storm of a man.
That one scene in the kitchen was masterful and prepared the perfect scene in the bedroom. Without the confrontation with the old man, when Ennis was not placed at the depth of despair, thinking he lost Jack, not only physically, but also in love, the moment of truth, of finding those shirts would not be as powerful as it was, breaking down all the walls that kept Ennis locked up tight. The love that Ennis thought he lost, the love that he yearned for, burst forth in a moment that energized him, and perhaps for the first time in his life, he cried.
I don't know. Probably not! But the main damage had already been done and the rest of it were just different shades of life. But all dark!
I see the light that is Jack's Ma, the angel of compassion and gentleness, and the light that is Jack's love, the love that has no end, and the light that is Ennis' devotion, the faithfulness that is singular and true.
I have been told that I see the end differently than others, and I do not apologize for it, but I also understand others' point of view.

I hope you find peace in/from the movie, whatever you see.