Assuming he was brain dead, at least he probably didn’t consciously experience that pain, in spite of whatever involuntary bodily spasms he might’ve made on life support. The man you knew died painlessly at least, and I hope that brings you some peace.

And some vampire traditions have them with arithmomania, which is why to escape them you drop a large number of something in front of them (so they’ll be compelled to count every single grain of rice while you make your escape, for instance).

You’re thinking of CO, increased CO2 in the blood (which when dissolved becomes CO3+) causes increased anxiety and too much triggers suffocation panic. That’s why you can breathe in basically any other gas and just shut down calmly, but CO2 will have you scrambling.