Unfortunately it appears they didn’t use a system which extracted CO2 from the air supply.

This is problematic with a mask vs. a chamber the CO2 concentration within the mask would have increased.

That would mean CO2 would not leave the lungs.

So the CO2 probably did build up in his blood and he experienced a suffocation sensation as if he held his breath.

We know how to kill people humanely. That was not the way this was done.

Even if someone wanted to die, this would have been an inhumane thing to do to someone.