We had no clue. The demon inside of Trevor who took his life was a stranger to us. We may never know how he felt he had hurt this girl, but that was the negative thinking that lead him to punish himself. I don't think she knows, or if she does, she isn't saying. I suspect it was all in Trevor's head, and that he concocted this anger at himself, and perhaps unknowingly, took it out on all of us.
Depression does that to the most loving and gentle of people.
I speak from personal experience. I have been in such a state. I suspect it is either self-induced or brought on by extreme trauma. Either way, it is difficult to break without some active intervention or an extraordinary subconscious will to live. This is a state of constriction accurately depicted.