Here is one example of why.
Most of the home-grown/suicide bombers come from the poor regions in Pakistan. Bin-laden was living inside of Pakistan for who knows how long, not to mention he lived next to a Pakistani military base in a big house. The worlds "most wanted" "most dangerous" terrorist living in a country that has plenty of nukes, most likely supported by military figures whether that be officially or unofficially. Pakistan has nuclear weapons, Pakistan is where the majority of home-grown terrorism actually come from. Who do you think is funding & weaponizing the groups loyal to the Taliban within Afghanistan right now? It's their connections from Pakistan....oh and we're sort of doing that as well...by leaving munitions at abandon coalition bases deep in Taliban territory.
What I am basically trying to say is...yes...there are some crazy people out there. But not as crazy as you might like to imagine. Iran has an extremely long history of self preservation. Are there people who'd like to see Israel and the US disappear from the earth? Of course there is...just like there is people from the US and Israel that would like to see Iran disappear from the earth.
Think of Iran as the cocky, smart-arse troll. Deep down as much as they'd like to see a nuclear attacks wipe Israel or the US from the face of the earth these guys know deep down that is simply not a viable course of action. Why? The US (mainly the US) and Israel have nuclear weapons, but they don't just also have them, they have more of them...bigger ones and they have a much greater ability when it comes to actually using them.
Look at the US reaction to September 11th. Around 3,000 civilians murdered. 2 wars, trillions of dollars.
Now think of the US reaction if a US city or US ally gets hit with a nuclear weapon? Iran knows it's going to get obliterated from the face of the earth, regardless of if they're completely innocent or not. The US government wont take it's chances, and with all the media hype up around Iran...of course all of the public is going to support nuking Iran ASAP regardless of the evidence.
If anything, Iran is the last person that would want anything to happen, they know they're first in the firing line if anything happens.