I hate Iran, and I fear that irrational state's impeding acquisition of nuclear weapons.
However, we cannot justify attacking them just to prevent that acquisition. Self-defense is the only righteous reason to use violence against others.
Might Iran use those weapons against us? Sure, they might. But that's not enough to justify killing them. Such a meager justification would legitimize attacks on anyone possessing any kind of weapon--See that guy with the steak knife? Sure, he's eating steak now, but he *might* stab me later, so I'd better shoot him now.
I say this with a grimace, for I really hate Iran; but they have as much right as anyone to possess such weapons.
Only one country in the history of the world has used nuclear weapons against another country, and that is the United States of America, against Japan (twice) in August 1945. This was done to prevent the need for an invasion of Japan, that would have resulted in a much greater loss of life (for both sides), but it was also done without adequate warning. Also, take a look at the history--the second bomb was unnecessary. We just wanted to show the Soviets that we had more and were willing to use them.