O.k. I get it. You will never change your mind about Brian, understood. I'm not emotionally involved in the story, so I'm just chatting.
I will put this idea with you, since I think it's possible. Let's just say, hypothetically that Brian was a little immature with some minor mental disorders, but otherwise about average in most other ways. He really loves Gabby and has some crazy emotions for her, but she keeps hitting him, and nagging non-stop for one to many days?!?! (More theories) Brian's emotion finally gets the best of him, and screams, SHUT THE Fork* UP, grabs her by the throat, and keeps yelling stop acting CRAZY YOU STUPID F----- B-----! By the time he comes back to his senses, Gabby isn't breathing, and he panics. The rest is history.
Let's say, this is what happens for discussion purposes, does this change anything?