What Paul Pillar says. We can live with a nuclear Iran, because their leaders are evil, not insane:
The simple argument is that Iranian leaders supposedly don’t think like the rest of us: they are religious fanatics who value martyrdom more than life, cannot be counted on to act rationally, and therefore cannot be deterred. On the campaign trail Rick Santorum has been among the most vocal in propounding this notion, asserting that Iran is ruled by the “equivalent of al-Qaeda,” that its “theology teaches” that its objective is to “create a calamity,” that it believes “the afterlife is better than this life,” and that its “principal virtue” is martyrdom. Newt Gingrich speaks in a similar vein about how Iranian leaders are suicidal jihadists, and says “it’s impossible to deter them.”
The trouble with this image of Iran is that it does not reflect actual Iranian behavior. More than three decades of history demonstrate that the Islamic Republic’s rulers, like most rulers elsewhere, are overwhelmingly concerned with preserving their regime and their power—in this life, not some future one. They are no more likely to let theological imperatives lead them into self-destructive behavior than other leaders whose religious faiths envision an afterlife. Iranian rulers may have a history of valorizing martyrdom—as they did when sending young militiamen to their deaths in near-hopeless attacks during the Iran-Iraq War in the 1980s—but they have never given any indication of wanting to become martyrs themselves.
I think Khomenei would've been most likely to be an insane-from-our-perspective fanatic, and he negotiated an end to the Iran-Iraq war because it was in the interest of his regime's continued power.
Of course this doesn't mean evil leaders won't be aggressive or take risks. Hitler attacked the Soviet Union when he hadn't finished digesting Western and Central Europe. OTOH, there's no way he would've attacked the USSR if the Soviets already had nukes. It is possible to reason through how an evil leader thinks. If Iran gets nukes, and I don't even know how determined they are to get nukes as opposed to nuclear capability, it would be insane to give them to terrorists.
I think warmongers accuse enemies of irrationality to keep the rest of us from using reason ourselves. If our enemies are insane, our only choice is to overcome them. We heard the same stuff about Saddam, with no evidence afterwards that he was insane. I suppose it's not impossible to encounter an insane enemy, but overuse of that argument, and Iranian history, suggests it's unlikely here.