I believe it’s been established that he had smoked meth that night and had a pattern of use prior to that. So that, being blind in one eye, and some drinking earlier would do it. Amphetamines of any kind are also very good at making you appear a lot less drunk than you are and personally speaking, the threshold for becoming convinced that someone setting off fireworks or like a river running is really gunshots or someone creeping after you is a lot lower and I can see him panicking and ending up dead from natural causes.
I remember a few points where I would be washing dishes and appearing very much alert and energetic and non-drunk to people around me and in a lot of ways was still able to form coherent thoughts, but also was convinced someone was trying to break in so I wouldn’t let myself leave the room because that’s where the knives were and all my thoughts would end up getting slightly paranoid and warped even though I could still do a lot of “normal” things. His parents mistook “tweaking enough to be animated, impulsive, and paranoid but not enough for someone relatively naive to recognize some kind of amp was involved” for sobriety. All of the moments in the call (calls? I only read one transcript not sure if that was 911 or parents or to who) where he begins to become progressively more unnerved about random sounds and convinced something bad is going on makes perfect sense. In my experience it’s a kind of paranoia where you’re totally fine and the smallest thing plants the seed in your head and you immediately spiral out even if all of the time leading up to that you were perfectly “normal”.
The “oh fuck” at the end was likely him tripping and his phone hitting the ground or even him falling into a body of water. Amphetamines will literally keep you alert for days and give you (at least perceived) super strength, energy, and endurance. He could have easily walked 10+ miles in some distant direction before succumbing. I feel like I could easily see myself seeing tail lights in the distance or a single gas station and thinking that’s where the “city” starts and walking for over an hour while being so fixated on thinking about some random thing that I don’t even notice time passing or the distance or the fact that I’ve probably changed course a few times anyways and am more or less walking at random at that point. And then when you do realize you’re fucked you’re probably still either completely incapable of feeling like somethings wrong or you’re so cripplingly panicked that you start doing stupid shit.
This case isn’t mysterious once you take tweaky paranoia into account. They just haven’t found the body yet.