LaBoeuf's death in the original film. Worst of all Rooster had to simply leave him there without a proper burial due to having to rush Mattie to a doctor to save her life.
The entire ending sequence in the 2010 version. When Rooster was forced to mercy kill Little Blackie is where it starts to sink in. That's when you start to think back on all the other dead bodies that he and Mattie have left in their wake. Then you realize why 90% of the score has been Christian hymnal music. Then the hopelessness of it all finally dawns on you and you just loose it (or maybe that was just me).
Oh Jesus no, not just you. The scene where Rooster and Mattie ride back through the canyon and there are those long, lingering shots of the Lucky Ned Pepper gang lying dead on the ground, and then when Mattie pleads with Rooster not to kill her horse.... the movie had been very good up until that point, but those last few scenes just cemented it in my mind as the best film of the year. Now I can't even listen to the soundtrack without getting sentimental.
Moreover, Mattie screaming for Rooster not to shoot her horse, despite the fact that he's clearly done for. This from a girl who has only shown calm and poise in the face of numerous difficult obstacles. And then the scene afterward, where he tries to pick her up and she fights against him. Guaranteed to break your heart.
Just the entire last 10 minutes of the film.
"Time just gets away from us."
One of Rooster's last lines in the film, if not the last after he falls to his knees in front of the trading post after having run a significant distance overland carrying Mattie after he mercy-killed Little Blackie.