Recently finished Speer's memoir, Inside the Third Reich. I'll have my own comments on the chemical weapons issue, but in case they're useless, I'll separate out a piece of history from late 1944 (at pp. 413-414 of the paperback):
Hitler, to be sure, had always rejected gas warfare; but now he hinted at a situation conference in headquarters that the use of gas might stop the advance of the Soviet troops. He went on with vague speculations that the West would accept gas warfare against the East because at this stage of the war the British and American governments had an interest in stopping the Russian advance. When no one at the situation conference spoke up in agreement, Hitler did not return to the subject.
Undoubtedly the generals feared the unpredictable consequences.
Hitler, btw, had been temporarily blinded in World War I from a British gas attack.