There is one very compelling reason Hitler never seriously contemplated the use of poison gas against the Allies.   As a corporal  during the Great War, he suffered temporary blindness due to a British gas attack.  Like the (somewhat unjustly) maligned British Prime Minister Neville "Peace for our time" Chamberlain, even a drugged and often out-of-touch-with-reality Hitler understood how utterly catastrophic a general use of poison gas would be, especially if directed against civilian population centers such as Berlin or London.

As a side note, I had no trouble whatsoever understanding  Sean Spicer's meaning when he made his now notorious contrast between Bashar al-Assad and Adolf Hitler.  He obviously meant the deployment of chemical weapons on the field of battle - not the use of agents like Zyklon B on helpless Jews, gypsies or other Untermensch in death camps.

Histrionic politicians taking Spicer to task such as Nancy Pelosi could benefit from an evening with a good history book.