How did the Axis and Allied powers resist the temptation to use nerve gas during WWII?

Nerve gas was such a devastating and fearful weapon during WWI, how did it come to p that, even in the desperation of such a mive conflict, neither side decided to use them? I'm pretty sure that both sides actually HAD them, but why didn't they use them? (I'm glad they didn't, obviously). Additionally, is there any lesson in this about the use of chem/bio weapons or even nuclear weapons today? Thanks for your answers.