It's a quirky aside to a grim question, but the Germans were largely and deliberately cut off from helium supplies by the United States. Helium was produced in bulk as a byproduct of natural gas production, and US Secretary of the Interior Harold Ickes ordered that no further helium should be sent to Germany after the Nazis took power. The idea was to rob the militant German state of a potent symbol (zeppelins) that was also linked to the wartime experiences of World War I. The result was that the Germans used hydrogen in their zeppelins, which, of course, lasted until the destruction of the Hindenburg in 1937. (By 1940, the Germans had disbanded all rigid airships and re-used the steel for other purposes.) Which is to say, helium was definitely not an option, from just a supply standpoint.

See Martin L. Levitt, "The Development and Politicization of the American Helium Industry, 1917-1940," Historical Studies in the Physical and Biological Sciences 30, no. 2 (2000), 333-347.

I don't know about nitrogen but nitrates were an intensely important strategic material (they were used for explosives and fertilizer and had to be entirely synthesized from scratch because Germany did not have access to natural sources). So I doubt they wanted to spend it on this sort of thing, if they could use other gases. But I am just gesturing here; I don't know what nitrogen production entails.

The Germans did use carbon monoxide — which is relatively easy to produce, using a gas-powered engine — for their earliest mass executions as part of the Aktion T-4 program ("euthanasia" of the disabled), starting in 1940. They were judged too slow and unreliable compared to the cyanide gas method, though it still continued to be used at some sites for its simplicity.

Part 1

So, for fear of getting backlash again for this, you do realize that you just asked about questions about making mass genocide more "humane", right? Basically wrapping your question about why the Nazis used the methods they used in a suggestion about how to make the mass killing process better. Forgive me for saying this, but this whole questions speaks not a lot of empathy for the victims of said process and what empathy is there is reserved for the perpetrators. I mean, that's a bit... lacking in caring and empathy for people who died gruesome deaths, to put it mildly. I mean, if you are interested in why the Nazis used the gassing methods they did use, why phrase it in this manner...

Content Warning for the graphic discussion of Nazi cruelty

Anyways, how the Nazis came to use the methods they used is mainly predicated on factors such as logistics, cost, and "effectivity" with humanity simply not figuring into it. For them, genocide was a question of how to effectively kill large numbers of people with the effect this had on its victims simply not figuring into it. These were people who killed babies by bashing their heads against walls. The humane treatment of victims was simply not a category that figured into their thinking. It is imperative to understand that they did not regard the people they killed as human. "Vermin" and "sub-human" were they categories in which they discussed their victims and in accordance with that they behaved as if they didn't encounter other human beings in their victims.

And this also goes into the behavior and impact taking part in these operations had on the perpetrators: While the topos of excessive alcohol consume or stress sometimes appears in official documents (specifically by Himmler, notoriously not involved in the killing he ordered himself, after visiting a shooting site in Belorus), most of the evidence we have suggest not that taking part in killing operations traumatized the perpetrators. The opposite in fact, killing and cruelty was frequently and massively turned into sport and entertainment for these perpetrators.

As Michael Mallmann relates in his article "Mensch, ich feier heut den tausendsten Genickschuß". Die Sicherheitspolizei und die Shoah in Westgalazien (Man, today I am celebrating my 1000th neck shot. The Security Police and the Shoah in Western Galicia): Acts of cruelty and brutalization were not random but for many security police and regular police units as well as for some Wehrmacht units, an essential part of carrying out the so-called "Final Solution".

He is quite explicit writing:

It was not the oft-cited "following orders" that marked the local reality of the Shoah but willingness to initiate measure, lust for killing, and the complete erosion of any sort of semblance of civilized values.

He goes on to further describe that the security police units in Western Galicia experienced "entertainment value from killing" citing examples such as putting explosive in the Jews' bags and then letting them force them to do a footrace with the police men standing near by laughing as people would blow up; forcing Jewish girls to get naked and using their buttocks for target practice; or when one of them shot their 1000th Jew having a big party for him (hence the title of the article).

The security police in Western Galicia killed at least 20.000 Jews and everyone of these executions involved something like the above described. And there are Wehrmacht unit with similar stories. And while in those cases a lot was up to the individual unit's commander, German soldiers and police men committing individual acts of random horror was an incredible common phenomenon during the Holocaust.

A similar dynamic is observable in the camps: Rather than becoming mental wracks, these perpetrators reveled in what they were doing. One such example is cap tossing. Prisoners in the camps had to wear caps as part of their uniforms. Guards would take these caps and toss them in an area where prisoners were not allowed to go and then order the prisoner to retrieve the cap. If the prisoner went, they'd shoot them in the back, if he didn't go, they would shot them for obeying orders. How we know this is because shooting a prisoner for either escaping or disobeying order would net a guard a few days of holiday they could retrieve at any point during the year.

Or take the example of the Boger swing, a torture device originally invented in Auschwitz by a guard named Wilhem Boger that found a quick spread throughout the camp system. As Boger's secretary described the device:

It was a meter-long iron bar suspended by chains hung from the ceiling... A prisoner would be brought in for “questioning,” stripped naked and bent over the bar, wrists manacled to ankles. A guard at one side would shove him—or her—off across the chamber in a long, slow arc, while Boger would ask “questions,” at first quietly, then barking them out, and at the last bellowing. At each return, another guard armed with a crowbar would smash the victim across the buttocks. As the swinging went on and on, and the wailing victim fainted, was revived only to faint howling again, the blows continued—until only a mass of bleeding pulp hung before their eyes. Most perished from the ordeal--some sooner, some later. In the end a sack of [sic] bones and flayed flesh and fat was swept along the shambles of that concrete floor to be dragged away

Auschwitz survivor Jean Amery has also written about being subjected to the Boger Swing while waiting to be deported to Auschwitz in the Breendonk Concentration Camp in Belgian although in his description, he was not hit on the buttocks but rather the guards beating him with an iron rod aimed for his genitals.

The point I am trying to make here is that when people like Franz Stangl, the commander of the Treblinka and Sobibor death camps, talks about how hard it was for him to gas hundreds of thousands of people and describing the mental harship of that process when in prison and talking with journalist Gitta Sereny for her book Into That Darkness: from Mercy Killing to Mass Murder, a study of Franz Stangl, the commandant of Treblinka, he is not only pitying himself instead of his victims, he is also disingenuous since from all what we know from the sources Stangl and his fellow perpetrators rather than suffering under their "task" were constantly coming up with new and inventive ways of how to humilliate and denigrate, beat and hurt, and torture and kill their victims better.

Consequently, the vast majority of perpetrators whether in the camps or as part of the shooting squads might have found the killing process unpleasant or even relished in it but were far from a point where any humanitarian consideration or thought was given to the massive suffering of their victims who languished beaten and starved and were murdered in gruesome, cruel and horrible manners.

This is what humans are able to do to other humans: Turn their murder into sport, into entertainment, revel in their suffering, laugh at their gruesome deaths and dream of new and more inventive ways to inflict pain on them. And here also lies the answer to your question: The reason the Nazis used the methods they used for mass deaths were questions of logistics, cost and "effective" and balancing those factors; not making the killing process more humane – even if Himmler occasionally mentions this, it just isn't reflected in the sources of the people making the on the ground decision on how mass killings are conducted.

So, I have written about the evolution of the Holocaust before but in essence the first major killing programs of the Third Reich, which would provide a lot of experience and input in terms of how to kill large populations of people were developed in the so-called T4 action, the mass murder of German handicapped and mentally ill people, and in Poland with mass shooting in the course of the so-called Intelligenz Aktion, the mass killing of Polish intellectuals, politicians, and people of public standing.

For T4 the first gassing methods were developed. On recommendation of the Kriminaltechnisches Institut der Sicherheitspolizei (KTI, Criminaltechnological Institute of the Security Police) the T4 gas chambers in the six centralized killing facilities used bottled Carbon Monoxide. The reason why was that SS-Untersturmbannführer Albert Widmann of the KTI had conducted animal experiments to find out which gas that was relatively cheap and in plentiful supply was the best to gas a large number of people in a reasonable time and at little cost.

Reconstructed from his own testimony as well as the testimony of other involved, the State Court Ulm summarized as follows:

The debated methods ]for mass killing] were, among others, Morphium, Scopolamin, Zyklon B and Carbon Monoxide. Because the accused [Widmann] had already had experience with poisoning by CO-gas and written a report about it, he suggested – potentially after having conducted experiments on animals [a fact later confirmed] – to Brack to use pure CO-gas to kill the patients. He suggested to implement the practice of funneling the gas into the sleeping quarters at night.

The latter part was obviously not implemented because it turned out more efficient to just construct a separate gas chamber with a ventilation system and a funneling system for the CO gas that at this stage came from cans.