Will the US pull an IRAQ if Iran refuses to cooperate with UN demands?

Another question I have, which maybe reflects my naivete: Is Iran having nuclear weapons capability something that is threatening enough to world stability that the United States should forcibly prevent it from happening? What is the liklihood that if Iran had nuclear weapons that they'd actually use them? And why are they so determined to have them? And why does their having them make everyone so uneasy?

From the BBC:

The House Intelligence Committee report says: "Iran is a serious security threat on which the United States needs better intelligence."

If I could choose between walking down a street where people carried guns and walking down a street where people carried no weapons, I'd choose the street without weapons. A world with fewer weapons that could kill me is better than a world with lots of weapons that could kill me. The obvious problem is that we have guns but we don't want other people to have them. Why is Iran a serious security threat? What might they do to us that has us so on edge?