Iran is worried about the Soonies not about Israel. The war between the Sonnies and Shiites has been going on for more than 600 years and most likely will continue as long as we live. An H Bomb on Israel will make Iran loose its defense ability and will be taken over by the Soonies who are approx. 85% of the Arab world.
Firebomb thrown during demonstration in northern Israeli town of Umm al Fahm (Haaretz)
from the article: Barak to Haaretz: Iran won't drop nuclear bomb on Israel