Sheikh Hassan Nasrallah is so critical to the success of Hizbollah, that if he's killed, the whole organization may fall apart. Although on paper the movement has clear lines of authority, and a defined order-of-succession, if you take a close look at Nasrallah's henchmen, you realize that most of them are pretty much the ordinary run of uninspiring Arab thugs. As none of them comes near Nasrallah in either brains or charisma (in a Moslem way), they would most likely begin squabbling among themselves if Nasrallah got killed. Iran has sent lots of advisers and technical experts to Lebanon, but none of these could expect to lead Hizbollah, largely because all Lebanese are touchy about foreign influence. Although the two decades of Syrian occupation favored Hizbollah, and thus Shias in general, it was still foreign occupation. And it's no secret that Syria considers large parts of eastern Lebanon as territory that should be part of Syria. Iran doesn't like depending so much on one guy, but they don't have much choice. Nasrallah, like most Arab strongmen, sees the elimination of any likely successors as an excellent method for avoiding getting deposed.