Any country that has nuclear weapons has the "doomsday button" and thus the power to end the world as we know it with a simple press of a button.
No one should have that kind of power. I think the question we should be asking isn't "should Iran be allowed nuclear weapons" and instead "should anyone have nuclear weapons?"
I think that too is a bit of a reach, based on quick assumption (philosophical aspect alert!
) let's say Iran blasts the hell out of Israel, why should that cause some kind of cascade resulting in ending the world (as we know it), well, okay, it would "end the world with israel in it" or "end the world with most of iran", but not thaaat much else.
Even an all-out between USA and China would affect mostly only USA and China, since China don't have any allies, and China in return have about 200 warheads, not exactly enough to litter the world in glowing mushrooms.
These were American targets btw, in Norway
In-case-we-flip targets
(and when I say "affect" I mean there is a big difference between radioactive rainfall, and obliterated capitals)
A more daunting confrontation would of course be Russia vs NATO, here we have the possibility to really shower the place in nukes, well - limited mostly to western world + russia, and again - limited only to Russia, since Russia is the only "antagonist" nuclear power with the capacity to cause significant damage globally
So
Let's sing the relax-song together, everyone, hand in hand!
Followed by the aw-fuck-it-More-nukes!-song afterwards!