I freely admit some of the US's foreign policy over the past half-century (during & after the Cold War) has been ill-advised and ham-handed in general, and specifically so in Iran's case. We have been the authors of many of our own woes. But beyond that, I believe it's telling that much of the rest of the Arab world is also worried about Iran, over more than just their sectarian conflict. I continue to think the Iranian leadership is capable of many things an average rational person would find horrendous that would be acceptable to the Iranian leadership through their rationalizing.
Certainly -- indeed, the very brutal crackdowns they've been enacting on dissension since the election last year has made that clear. But that does not mean they are not rational political actors. They are -- evil, brutal, tyrannical, but still rational.
And the fact that they are rational political actors means that, in my opinion, it is highly unlikely that they would launch a first-strike nuclear attack against any other state, nor that they would allow terrorists to use Iranian nuclear material to engage in such an attack. The leaders of Iran know full well that if they do that, that means the end of their regime (and of their lives); the world will not tolerate a nuclear attack from anybody, especially not a second-rate power. That's why I think while they may well try to push it to get themselves the bomb so as to possess their own nuclear deterrent, I think it's highly unlikely they'd actually use it.
But, as I said before, I'm scared of the North Korean nuclear program. Those leaders are not rational political actors, in my view, and may well decide to do something that's not in their own best interests.