[DiogenesLamp]

So all government needs to do to ban Z is declare it "an existential threat to the existence of government"?

If "Z" constitutes a substance of sufficient deadliness, (Such as VX Gas, Anthrax, Fissionable material or mind altering drugs) then yes.

Do you think governments should tolerate substances which can end governments and kill millions?

[ConservingFreedom]

VX Gas, Anthrax, Fissionable material or mind altering drugs

I can't help but notice that only one of those items is self-administered. To people who care about individual rights, that makes a great deal of difference.