3 Iranian nuclear scientists have already been killed by car bombs and a fourth narrowly escaped assassination. The question is - who's killing them? The US or Israel? A nuclear Iran massively shifts the balance of power in the Middle East and makes it harder for us to accomplish whatever happens to be our interventionist goal du jour. That said if we don't police the world who will? China? As much skepticism as I have about the success of our past interventions I'd rather live in a world where we're bossing people around instead of China.
I found one of the best explanations of Iran and its goals in this podcast: