First off, I hope this question is not too offensive. Discussing technicalities of a genocide will certainly disgust some. I am in no way trying to condone nazi crimes. I am also not sure whether it makes sense to search for rational thought in genocide. Here goes anyway:
Nazi death camps used shower heads to introduce a gas into the gas chambers, thereby killing people. The gas used was Zyklon-B, an industrial product produced by a single supplier, and likely relatively expensive. It also meant that the gas chambers had to be aerated for a number of minutes before soldiers or forced laborers could enter the gas chambers to drag out the corpses.
Why didn't they simply use CO2? It's a byproduct of basically any fire. It's cheap and could have been produced on-site trivially. It's also part of normal air and only toxic in high concentrations, likely meaning less danger to soldiers.
I cannot remember when or where I heard or read about this, so take it with a grain of salt. But I recall hearing something about the people who had to remove the bodies from trucks where people had been gassed with CO2 finding the corpses so gruesome and agonising, as opposed to more peaceful looking bodies of people killed with other gases, that they stopped using it. Presumably the difference was because of how slow or quick the death was.
I doubt they would have cared the tiniest bit later on when it was other concentration camp victims who had to clear the bodies.