If you don't believe Iran will use nuclear weapons once they have them, you haven't been paying attention. Their leadership has continually called for Israel's destruction. Ahmadinejad pulled every trick in the book, including murder of Iranian citizens in the streets to secue his sham "election" a couple years back. He and others in the leadership are "Twelvers" who wish to hasten the return of the Madhi. For that to happen(in their view), chaos must reign and the region must be washed in blood so the Madhi can return to restore peace. If you have a group like that in charge, why would you dbout anything they wouldn't use every method at their disposal to bring disaster to the region?
12 Iraqi troops, militiamen killed in multiple ISIS suicide attacks (AP)
from the article: IAEA report: Iran has been working toward nuclear bomb since 2003