So, does not signing a stupid treaty make the current situation justifiable?
No, but Iran is violating a treaty they signed while Israel did not.
What I am saying is that the main problem with Iran acquiring Nuclear weapons is that they would violate the non-proliferation treaty and would have lied to the whole international community, since they have continually claimed that they are not trying to acquire Nuclear weapons. Hence, if Iran were to acquire Nukes, they would render International law absolutely useless.