Well let's consider the equivalence you suggest:
On the one hand we have a tyrannical theocracy the leaders of which constantly rant about the destruction of Israel, and who are actively seeking the means to accomplish such a goal.
On the other hand we have the only democracy in the region, who have nukes (and no one is the least bit afraid they will use them as a first strike option) whose people have a history of paying the price for the world ignoring nations rhetoric about eliminating them.
Israel knows only too well that they cannot rely on any nation (including the US) for their continued survival, and with Obama as the president of the US, their reliance is that much more tenuous.
They know pogroms and the Final Solution far more intimately than the rest of us, and they have every reason to believe such atrocities can happen again; in the here and know.
Clearly a unilateral strike on Iran is highly problematic for them or they would have already launched it, but they can only wait so long before they have to commit to action.
Unfortunately Obama's assurances that his Administration is more supportive of Israel than any others before it is seen as hollow by Israel...as it should be.
Here is what happens if Iran gets nukes: The region becomes a nuclear tinderbox with both countries trying to anticipate when the other will launch their nukes. It will be the Cuba Missile Crisis squared.
Only this time someone will blink and millions of people will die.
Sadly, there is no shortage of folks who will, even after such a catastrophe, be trying to cast blame...and largely on Israel.