You said it,
@Lavender Moon -- it's always good to have one sober person (I don't drink alcohol) to drive (of course) and to remember where the car is parked. Most of us learn that, sometimes the hard way, earlier rather than later.
After Swigs, did his friends just go their separate ways, or did Brian leave them in the bar? What? He must have been by himself when trouble happened... Did he get in trouble when he got to his car -- did he make it to his car? Was he followed?
I'm thinking we'll never know much more than we know now unless we see the autopsy results -- his BAC and any injuries that look like foul play.
Such a sad, sad thing to happen...