I guess it depends on who is watching the film, and their own circumstances, when it comes to feeling the pain of the characters, or who "endures the most or the sharpest pain"
I can still remember seeing BBM for the first time, having not read the story, and was so elated with the reunion scene, that it turned out the way we were all hoping it would, and THEN Alma looked out the door, and the change in emotions I was feeling, from happiness and elation to a gut wrenching OH NOOOOOOOO....well it nearly made me throw up. It was completely unexpected, and sent my thoughts into a tailspin. Such an abrupt end to the euphoria of the reunion scene. We had got so caught up with the excitement for the boys, that we never gave Alma a thought, and then there she was, in a way, spoiling it for us, like the third wheel, we didnt want her there, but she was there.
I think Alma was feeling just like we were, the build up of the day, seeing how excited and restless Ennis was, seeing the joy and excitement when Jack arrived, she wanted to be part of that excitement and then competely unexpected that joy turned to bewilderment and devastation.
Alma was the innocent party in this threesome, she did nothing to promote it, and could do nothing to prevent it. And in that torment in her life, she also had the two girls to care for.
Everyone in this movie went through their own devastating pain.