With nukes, no one can stop the government from running people over with tanks, shooting questions, "purifying" Iran and such. You think the regime wants to liberalize? They chant Death to America/Israel in Parliament.
I dunno who you think the Iranian regime is. Perhaps you didn't notice Syria, Hez and Hamas resulting from them. Perhaps the terror attacks in Iraq and around the world didn't make your radar.
The people of Iran would become animals, property, overnight. Syria, Hez and Hamas would become much more horrible things.