Do we really want an Iran that funds terrorism around the globe to have the trump card of nuclear weapons? Right now Iran is mostly an annoyance, but an Iran with a capable nuclear weapons stockpile, and delivery platform is a serious, destabilizing problem that is more akin to lymphoma than to a bar brawl.
Israel can be a PITA, and I'm no fan of many of their actions over the past 40 years. However, once in awhile they are right. They were right about Iraq, and they are right about Iran. The problem is, they truly lack the capability to tackle the problem on their own. The United States is a little better off in that regard, but not a lot. We gave up the strategic position of being able to use Iraq as a spring board, and we don't have much capability in Afghanistan (the topography is very poor for any ground based invasion). The only weapons in our inventory capable of delivering the blows needed would be B2's, nuclear weapons, or a sustained ground invasion. Fighter-bombers just doesn't have the payloads necessary to truly destroy hardened facilities (meaning Israel really can't do a lot here on their own without a cooperative neighbor that shares a border). [I've seen some very interesting photos of facilities the United States targeted in Iraq -- one of them was hit with more than 60 cruise missiles, and bombed with more 20 1000lb bombs. The damage on the inside was rather unimpressive. If Iran has hardened its nuclear facilities -- cruise missiles, and non-nuclear weapons launched from fighter-bombers are not going to be enough to do the job.]
Your argument is basically: We should give up the idea of nuclear containment because the world is a lot safer, and less likely to blow itself up if everyone has a bomb -- including the truly crazies of the world.
My argument: Iran has funded all kinds of terrorism in the world, and has a track record of being a persistent thorn in the side of the United States, and much of the civilized world. An Iran with a nuclear trump card is far more likely to act even more poorly, and to spread nuclear weapons all over the region, and beyond. They might also spread chemical, biological, and other types of weapons all over the world since they would be beyond the reach of the United States to deal with barring an extremely costly exchange of weapons I would prefer don't come out of storage.
We can deal with a little bit of pain now with some air strikes, or even a full scale invasion (though I would hope it doesn't come to this) --- or we can deal with a world where every single conflict has the potential to go nuclear, and the next set of terrorists may be wielding nuclear materials.
I'm sorry the Iranians have poor leaders. I'm sorry the United States has an incompetent president. I'm sorry the Palestinians got a raw deal, and I'm sorry that Israel has some poor leadership as well. I have anguish for the Iranians that will have to die to fix the problems their leaders are creating. However, I'll take a world without a nuclear, chemical, and biological threat on every street corner over one that has it. Its a lot cheaper to deal with the problem now than to try to deal with it later; not just for us, but for every citizen of the globe.
Sometimes playing "world police" IS in our national interests, and in the interests of the world at large.