Iran openly says that it intends to “annihilate Israel”. Why do they want to destroy Israel? They believe that Israel is evil and destroying Israel will catapult the world into the messianic era with the return of the two Islamic messianic figures of Jesus and the Mahdi (whom Twelver Shia Muslims identify with the hidden twelfth imam in occultation). What do they plan for “the Great Satan”? We can only guess. But Iranian nuclear weapons capability would offer Iran impunity for attacking the US with other WMDs (chemical, biological and radiological) as the US would not dare retaliate with nuclear weapons for risk of an Iranian nuclear counter strike. There is no evidence that Iran intends to destroy the United States, but its intentions towards it are no doubt malign. Iran may well be planning a genocidal WMD attack against New York City because of its large Jewish population. This would be logical be logical from the perspective of Tehran’s Anti-Semitic worldview.
From a rational strategic perspective would destroying Israel with a nuclear attack give Iran hegemony over the Middle East. That Israel could and would wipe out major Iranian cities does not seem to bother Iranian decision makers since they apparently assume that Allah is on the side of Iran and Allah will therefore logically protect Iran from Israeli nuclear retaliation. This of course stretches into the realm of unreason. Another Iranian nuclear attack, this time against the Saudi capital of Riyadh would give Iran control over most Gulf oil and transform Iran into a global superpower that could dictate policy to Europe and Asia alike.
Iranian nuclear weapons would no doubt be a gamechanger on the regional and global chessboard and Iran fully intends to use its nuclear weapons. Iran is an openly Anti-Semitic, totalitarian regime which fully intends to perpetrate a second Holocaust. There is no doubt that its Anti-Semitism plays a major role in its desire commit genocide against the Jewish people. However, its motivations are a mix of Anti-Semitism, apocalyptic Islamic theology and purely strategic considerations of attaing regional and global hegemony. The Iranian regime believes that destroying Israel is the pivot in its eschatological process of attaining regional and global hegemony. Once Israel is destroyed will there be no one to defend Riyadh from suffering the same fate as Tel Aviv, namely nuclear Holocaust. Once Iran controls Israel will Iran attain regional hegemony and once Iran controls Saudi Arabia will Iran attain global hegemonist superpower status through the extortionist power of Gulf oil. With Saudi weapons will Iran subsequently take over the other Gulf states as well.
Do the great powers of P5+1 not see the danger? They probably do or at least so in theory but they assume that if it comes down to it will Israel strategically degrade the Iranian nuclear weapons program and thus resolve the problem for them. They are right to assume that of course and are happy to have the Jewish state as their life insurance against Iranian hegemonist ambitions.