I think a lot of people either forget or are totally unconscious about Iran's leadership's eschatology or constant barrage of "Death to America".
They see it as only an Israel problem, seems like Muslim lobbying has been successful at putting forth the idea that they only want to destroy Israel, and so it's not our problem as well. Iran doesn't just hate us for supporting Israel. They hate us because we backed the Shah, and the Islamists hate us because they hate western culture, seeing it as degenerate and "the Great Satan". Israel is only "the little Satan". The US in particular is "the Great Satan".
Twelver eschatology in particular, believes that they have to put the world into war in order to bring the Mahdi (their 12th Imam), and in particular they must war against the Jews. Their eschatology believes that unless they kill the Jews, the day of Resurrection is postponed.
What if there was a sect of Christianity that believed they had to kill all other denominations of Christianity to have the second coming of Christ? Would you want them to be in control of a country? Would you want them, in control of a country, possessing Nuclear weapons?
That's the equivalent of why Iran with nukes is so dangerous. They don't want nukes as a defense deterrent. They want nukes to strike their enemies and usher in the last day and resurrection.