I've cracked this case.
What happened is they met this lesbian couple where they worked at a restaurant a few days before the lesbians went camping. The lesbians told them where they were going to camp, so brian and gabby went to meet them there. gabby hates lesbians, and she planned to kill them with a gun all along, and didn't tell brian about it. brian hates lesbians too, but he didn't want to kill them. when they get to the lesbian camp, she pulls out a gun and kills them in cold blood. brian is like wtf, but he stays calm and acts like it's no big deal because he knows Gabby is crazy. for the next few days, he tries to tell her to seek help about this problem, tries to convince her that she was wrong for killing those girls, finally Gabby goes nuts and thinks that Brian is going to snitch on her, so she threatens him and they tangle a bit, then Brian, afraid for his life, kills her with the gun she tries to kill him with. then Brian wanders the tetons a few days freaking out until he decides to hitchhike out of there. he knows that everyone is going to think he did it now, so he's on the run.
