Why shouldn't Iran have a nuclear program?

Why are the US, Israel and their allies so determined to halt Iran’s nuclear program? The IAEA inspectors have failed to find any evidence of weaponisation since 2003 (the same time as Iraq, their major rival ceased to be a threat). Iran is also a signatory of the Non-Proliferation Treaty, and therefore is obligated to keep their nuclear program peaceful in intent.

More importantly though, what makes the US and Israel think they have the right to dictate the actions of another state? Even if Iran did develop nuclear weapons, they are certainly intelligent enough to only ever use them as a deterrent, and never actually deploy them. There is no “existential threat” to Israel, since the Iranian regime is not suicidal. As such, no threat is posed by a nuclear armed Iran, and no external state has a right to intervene.

There is also no chance of terrorists gaining control of nuclear technology in Iran, since they have done more to combat insurgency than Pakistan, which is both nuclear armed and an ally of the US.

So to reiterate the question, why shouldn’t Iran have a nuclear program? What ills could possibly result from them obtaining nuclear technology?