Well, this isnt about how YOU feel after wards. Its about what causes the least amount of tension/stress/pain to the animal.
"however moments later it is all removed by displacement.."
Moments later. Speak to anyone that was in a near fatal accident of any kind and they will tell you that seconds feel like hours. Moments later can be an eternity knowing that your suffocating. Make no mistake, they know when they cant breathe. Not to mention you STILL have oxygen in your body that keeps you alive for a few more seconds/minutes.
I understand for YOU its easy to turn on a valve and 30 seconds later see a dead snake and think "well that was quick and painless". But your not the one suffering.
Have you ever been under water to the point you were panicking for air? Well, snakes also use a much smaller oxygen percentage then we do and takes longer for the oxygen to fully dissipate in the body. Just because its not messy, doesnt mean its not stressful or painful.
Its not pretty, but smashing a skull and the brain cavity just might be the best FOR THE ANIMAL... Co2 simply doesnt make much sense to me given the nature of how reptiles utilize oxygen in the first place.