I watched the clip. Yes, it was a seemingly "peaceful" drift into unconsciousness, but it ended too soon. I'll bet that if the lack of O2 and increasing CO2 continued, the man would have experienced unpleasantness:

The guy didn't want to off himself. Did you watch through the part where they snapped the slate, and a crew member brought an O2 tank?

As he explains in the run up to that bit, it's CO2 that triggers the alarm response. He had a chemical in that rebreather system that absorbed the CO2, so there was no alarm response.

Dog pounds, as least in the past, used decompression chambers to kill dogs. There should be plenty of documentation on how the dogs respond, before they die.

You don't need a phlebotomist, to put people under with chloroform. Eons ago, I watched a "Nova" piece on 19th century research in anesthetics. The doctor doing the research would invite students over for supper, then the two of them would start huffing different chemicals. When they got to chloroform, the doc was commenting on how the smell was not unpleasant at all, quite nice, then they both keeled over. Chloroform is no longer used, but there are other inhaled anesthetics available.

Steve