For a while, there was generally an international consensus that nuclear weapon proliferation was bad, mainly because it was thought that the likelihood of using nuclear weapons would go up as more nations had them.

It was wrong for Trump to ditch the Iranian nuclear deal, but the original deal was a deal with most of the major nuclear powers including Russia and China. There was an international interest to prevent nuclear proliferation.

I don’t think it is right for the US to attack Iran regarding nuclear proliferation as Trump has been negotiating in bad faith, but that it is the idea where developing nuclear weapons is a bad thing which requires an international response comes from.