Carbon monoxide (CO) is a silent killer if the concentration is high enough. It was the first method of killing in the German T-4 program. The first gas chamber was hastily set up in Brandenburg castle in the late 1930s, after Hitler asked Drs Brandt and Wirth what was the most humane method for mass killing.
When the T-4 program "officially" ended (it kept on sub rosa) the carbon monoxide went east, first used in killing vans in Poland, using car exhaust fumes. It was a mess.
When the larger gas chambers were set up in Treblika, Maideneck, and Sobibor, carbon monoxide from a Volkswagen "bug" was used to kill hundreds at a time.
The concentration of gas was too low, and horrible pain resulted, certainly not painless. Auchwitz was the only facility to use cyanide gas (HCN), also with horrible results, as a pungent additive made the gas awful to smell.
There are problems with the use of lethal gas-- no matter which one is used, as seen with the seizures and struggling in the execution of Harding in Arizona a few years back. One North Carolina gas execution took nineteen minutes when I was younger-- a jillion years ago.
Although on paper all modern forms of execution claim to be painless when done right; they are not done right; they cannot be done right, except on paper.
As far as a deterrent is concerned, only in two situations is the death penalty a deterrent: (1) after the inmate is on death row, and (2) for those people who wouldn't kill anyhow, without the death penalty.
Dans le droit, comme dans l'amour, jamais jamais, jamais tourjours.
[In law as in love, never never, never always]
Charlotte NC USA