I think he's probably got some sort of SMI (severe mental illness) which he SHOULD be medicated for, but doesn't (i.e., his comment in the police video where he says, "but I don't take medication for it").
I also know that for some people with SMI, it is VERY common for them to self-medicate with other 'stuff' (alcohol, illegal drugs). I did a 6-month internship in college at a psychiatric care home and every one of the residents there had multiple instances of problems with drugs and alcohol being used in order to deal with their symptoms.
One of DH's college friends took a couple of years off from college because of an onset of bipolar disorder. When he became ill, the guy had manic episodes which were so bad that he became psychotic, would hear voices, and he'd do really insane and dangerous things...things which, to him at the time, seemed very logical things to do. The guy has now been happily married for many years and has 3 kids and is a high school math teacher....and he takes his meds religiously. Never misses a dose. As he put it once, "I never want to be that sick ever again like that. That was really scary."
What's happened to Gabby is very tragic. The truth of what really happened will eventually come out. Eventually SOMEBODY will find Brian...they will either find his body or, if he's alive and hiding somewhere, SOMEBODY will see him and think, "Huh, he looks an awful lot like Gabby's fiance/boyfriend who they're all looking for" and that person will report what he/she saw to the authorities.
I and all of us here can sit here and speculate until the cows come home but we just need to be patient...which is hard to do because it's an emotionally difficult situation and people want answers.
I'll tell you one thing - there's been enough weird stuff happening in that little area of Moab, Utah lately that I don't want to go anywhere near there any time soon. No offense to anybody here who's from Moab, of course.