observation
Seems a lot of people are focused on the reported normal respiratory rate. Just wanted to point out some things, irrespective of what the final cause of this accident turns out to be.
1. You close your eyes and relax - you already know the outcome. You have put the plane into a gradual descent, it will not structurally fail prior to impact, & you know nothing will affect the outcome until the end. Maybe the GPWS would jar you out of your relaxed state, but.... I still think someone who knew the outcome may not become overly excited if this was their intention.
2. He's pictured on the internet, in numerous photographs, running. Was he very physically fit? Might explain a lower baseline respiratory/heart rate.
3. Person took medication. Xanax? Alcohol? Propranolol? Opiate? The list is endless. Some, while not approved for flight, may have been prescribed while he wasn't flying and being treated for his mental illness (e.g. xanax or propranolol or another anxiolytic or antidepressant). And he might have taken something random that day. I assume alcohol or some other impairment would have been noticed by the other pilot, but he could have popped a boatload of pills at some point that only took effect late in the game.
If this act was related to depression, he may have needed something to decrease his inhibition. Lots of people who commit suicide tend to do this (from my experience as a pathologist). Alcohol is usually number one, but it doesn't rule out something else. Doubtful enough tissue will be identifiable for tox, unfortunately.
Pure speculation on my part. But thought I'd offer some other possibilities for the people that are placing a lot of emphasis on the normal respiratory rate. Happy if others completely disagree (us scientists like having our theories dispelled!)