WSJ: Armageddon and the Mahdi: How Ahmadinejad is giving Shiite Islam a scary name
points out the belief that the 12th Iman is the Moslem messiah and will return at the end of time to establish the ideal Islamic society is little different from the similar Christian belief based on the book of Revelations. And, indeed, the Moslems of the early Islamic era had Christian and Jewish teachings readily available to them when this belief was established. Apparently, one difference between Sunni and Shi'a Islam is that the former believe that the Mahdi hasn't come yet, while the later believe that he has, but then has been occluded for the last thousand or so years. And most Moslems, Sunni or Shi'a, have a similar view to this as do most Christians - that the end of time will come some day, but probably not in our lifetimes. Interestingly, the Mahdi is expected to come with Jesus.
Nevertheless, the president of Iran, seems to be more akin to those Christians who believe that the end of time, judgement day, etc. are imminent. And that is what is scary about him - that he might try to hasten it, by, for example, using nuclear weapons on Israel.