During the Second German War both sides expected the other to use poison gas. And both sides were ready to retaliate in kind for the first use of such gas.
So why didn't Hitler, especially when things got rough after Stalingrad use poison gas? It certainly was not because he believed only in an ethical war - and he was happy to use poison gas in other contexts.
There are three general views, and I incline to the last.
First, Hitler had himself been gassed and recognizing the inhumanity of the weapon forbade its use. With another leader I might accept this. Mustard gas, from the First War, was dreadful and relatives of mine died fighting for the Kaiser from gas perhaps delivered by relatives of mine fighting for the King. Certainly gas is a weapon of mass destruction and properly banned from use. But Hitler was not an ethical fighter. Indeed, he considered his enemies, at least some of them, to be less than human. And he had no trouble using poison gas on captive Jews, gypsies and Freemasons (among others). No, I don't see Hitler forbidding gas because of ethics.
Second, some argue poison gas is an ineffective weapon. There may well be something to say for that. Poison gases have always been very tricky weapons to handle and are unsuited to a moving warfare; as the blitzkrieg was. Still, a few canisters of gas on D-Day might well have been effective. But I grant the potential value of poison gas may have been limited; that may well have been a factor in it not being used by Hitler.
The third, and to my mind most convincing suggestion is that of the danger of retaliation. Hitler knew poison gas was not a war changing weapon. By the time Germany had a need to use poison gas the war was effectivly lost. The Allies were bombing anywhere they wanted with, maybe not impunity, but certainly implacability. Hilter also knew that gassing soldiers was ineffective, but if he used gas on Allied troops, the Allies would use it on his cities. Can you imagine what would happen if all of Berlin was covered in a cloud of Mustard Gas? It would be the end of the war right there.
Broadly put, use of poison gas can be deterred (even someone as evil as Hitler can be deterred) by a credible threat of retaliation.